(Disclaimer: All of the article below is based on my experience leveling a Sentinel. YMMV, as always.)
The Jedi Sentinel can be an interesting class to level, especially when it comes to picking a skill spec. The three skill trees all say "Damage," which doesn't tell you a whole lot, but after reviewing the talents, you can get the gist of them:
*Watchman *does its work in Juyo Form, lights things on fire, has Force Leap with a shorter cooldown and minimum range of 0, and is capped off with a 5(!) Focus ability that hits like a train. It also has some minor self-heals from its talents and from Zen. Watchman seems to be the "traditional" leveling tree, mostly because of these self-heals and ease of play (at least until 40, when you're trying to figure out how to shoehorn Merciless Slash into your priority list, but that's another topic entirely).
Combat focuses on hitting more often thanks to Ataru Form (both in the sense of bonus accuracy, and in terms of Ataru Form hits), contains an attack that lets you momentarily pierce all armor and is capped off with a Slash replacement that... causes you to do more Ataru Form hits. Really, the entire thing's built around maximizing Ataru Form. Even its Zen ability is based around Blade Rushing more (or Slashing more, prior to 40). Combat also, notably, has exactly zero survival tools outside of a way to avoid some AOE damage - it is the "glass cannon" spec. Unless you're towing around Doc or using one of the tanks, or wearing some extremely good armor, it can be a mite tricky to level as Combat.
And then there's Focus, the shared tree - although there are minor differences between the Guardian and Sentinel version of these. Focus seems like a real oddball at first - its work is done in Shii-Cho Form, which (thanks to talents) lowers a number of cooldowns, it has multiple talent boosts to Master Strike and Force Stasis, and it picks up some bizarre talents in the form of Zealous Leap (basically Slash on steroids with a 10-meter range) and Force Exhaustion (a DoT + progressive slow that ends with a big hit). When in Shii-Cho Form, Zen is similar to Combat's Ataru Form Zen, but with a slight difference: Slash is completely free, and also cleaves a nearby enemy. Its survivability comes from the inherent damage reduction of Shii-Cho Form (which increases from 3% to 7% when talented) as well as reducing the cooldown of Guarded By the Force to 1 minute.
Focus is easily one of the odder trees - it discards some abilities you gain from being a Sentinel (like Juyo Form) and focuses on things present for JKs from early levels, while also adding strange new abilities like a second leap and a DoT. So what's the big deal? You might want to think about leveling as a Focus Sentinel. Why? More after the jump.
Now, keep in mind this only applies to Sentinels (and, I suppose, their Marauder cross-faction ilk). The Guardian version of Focus lacks Blade Focus, and has somewhat less need for innate damage reduction due to wearing heavy armor. For the most part, however, this advice should be transferable - just remember that I really don't know anything about Jedi Guardians because I haven't leveled one. (Yet.)
So what are the pros and cons of leveling Focus? Pros first:
Innate damage reduction. Watchman offers 1% heals when your burns tick, and an extra 1% for you for the six ticks of Zen. Combat... hopes and prays it can kill everything quickly. Focus starts with 3% damage reduction, jumps to 5% somewhere in the 40s, and ends up with 7% damage reduction by level 50. It's at least as hardy a spec as Watchman (or more so, in my personal experience) - handy if you want to level with a companion whose name isn't Doc.
Highly suited for AOE. Mobs rarely come alone in SWTOR. Yes, Watchman has 2 talent points at the bottom that make Force Sweep and Cyclone Slash cheap and dirty. We'll be taking those first as Focus. They pair nicely with talents like Felling Blow and Singularity, which turns your Force Sweep into such a monster that you'll be hitting it in single-target combat too. There's also Shii-Cho Form's Zen effect, which gives you six free Slashes that also cleave a second target for 100% damage (and, once properly talented , turns Slash + Zen into a focus-generating machine - yes, you are refunded a Focus per Slash, despite not spending any!)
Simple and satisfying. Simply put, if you like simply killing the shit out of things, Focus will be nearly as good a tree for you as Combat or post-40 Watchman. Everything in Focus' PvE arsenal is based around two simple concepts: hitting like a truck, and making other things in your ability list hit like a truck. Additionally, it's not a complex tree to play - only Combat is simpler, within the same class, and there aren't really any timers to monitor or random events to watch for - the procs such as Felling Blow and Singularity are predictable as they come, so you really just need to focus on hitting the big-damage buttons until everything is dead.
More Master Strike, more often. I dunno, it's just a personal favorite of mine. It might not be worth using on bosses (it's seemingly not for Combat, and Watchman only uses it under specific circumstances), but it's just fun.
And the cons:
Lack of endgame/party play preparation. Granted, this may not matter to you. Right now, Watchman - as DoT-based specs tend to do - is mathing out as the top DPS spec for Sentinels. The argument could be made that leveling as Watchman would be the best way to get used to Watchman to perform better in operations. The truth of the matter is, though, that without a combat log it's still not 100% certain who's on top - hell, the Watchman/Annihilation community is even split over whether their top-level talent is worth a point - or whether that matters to you.
No self-heals. Focus' survival ability is very passive - Shii-Cho damage reduction - and doesn't have the self-heals of Watchman in Juyo Form. Its only other real boost is a cooldown reduction on Guarded By the Force. Of course, on the flip side, that damage reduction is always there, while Watchman is dependent on crits for its healing--and while you're able to guarantee six critical ticks with Zen, you have to have the Centering available to do so, and you need to have six ticks of burn damage ready to apply. It's probably six of one, half-dozen of the other.
Not a lot of cool visuals. Watchman lights people on fire regularly, leaps like a bunny, and their top-tier talent looks like you're whacking someone with two bolts of lightning. Combat is simply a flurry of blows - you are the fastest thing with two blades. By contrast, Focus seems almost methodical, and a lot of time is spent using a lot of the same abilities you're used to before you ever turn 10 - Slash, Blade Storm, Force Sweep, and so on. Zealous Leap looks neat, but it's very similar to Force Leap in terms of appearance. If you're one for consistency, this might be okay, but if you need some visual variety in your attacks, this might not be the tree for you.
The Singularity/Felling Blow combo is probably due for a nerf. I understand it's supposed to hit hard, but seriously, this is the hardest hitting thing in my arsenal, and I can pull it off every 18 seconds. Call me cynical if you wish, but it seems too good to be true.
Enough With the Bullet-Point Shenanigans, Show Us Some Skill Specs
Alrighty then, insistent reader, you asked for it! I'll start with the first talent point, then jump a few levels at a time. This is, as with most things I write, solely for PvE - a talent spec suitable for leveling by questing and running Flashpoints.
Levels 10-11: Oh no, where do I put my first talent points
In Watchman, interestingly enough. 2/2 Quick Recovery is simply too good to pass up, at this point or any point, giving you a cheap and easy tool to use on groups. At 12 you'll get Cauterize, and you can also use it if you like because at this point in your career, you really don't have anything else to spend the focus on.
Levels 12-22: In which very few things are put onto bars
Now we've got skill points in Focus. You'll get a few important things during these levels that you will promptly ignore as PvPe Focus: Juyo Form, Leg Slash, and Crippling Throw. You'll also get the occasionally-useful "oh shit" button of Call on the Force (which you will actually find in your abilities under "Heroic Moment: Call on the Force") and your very important interrupt. Additionally, if you're following this build, at 22 you will also pick up Zealous Leap. Here's a guide to using Zealous Leap: use it whenever you'd use Slash, then resume Slashing. ZL costs the same thing as Slash, but hits harder (although it doesn't have an increased crit chance). Starting next level, the advantage of this is that you'll also proc Felling Blow, increasing your chance to crit with Force Sweep by 100%.
Levels 23-31: The why-can't-I-Force-Sweep-more-often blues
Just kidding. If you could Force Sweep more often with our current talent build, it might be all you ever did. In terms of talents, this is one of those periods of time where you're just making things better - Slash and ZL hit harder, Force Stasis can't be pushed back, Master Strike is uninterruptible, and you now have both Felling Blow and Singularity. Unfortunately, the only thing you have to activate Singularity is Force Stasis, but it's really, really nice when you do - Force Leap into a group, Force Stasis one of them, then Force Sweep to do major damage to anyone around you. Proceed to beat in the faces of everyone who challenges you. As for abilities, a few important ones come in this range.
I mentioned Force Stasis; that's at 24. Speeder piloting follows right behind it at 25! Not combat-related, but a cause for celebration on every character. You also get Pacify at 26. There's very little not to like about Pacify - one-minute cooldown, 1-focus cost, and the target has a 90% drop in melee and ranged accuracy for 6 seconds. It'll save your ass against elites, and while it no longer works on Operation bosses, I'm happy to report it at least still applies itself onto Flashpoint bosses. PACIFY ALL THE THINGS
**Pommel Strike at 28 is the perfect follow-up to a Blade Storm or a superpowered Force Sweep. Disable Droid, also at 28, the only thing we get as far as long-term CC. Sorry.
You get Force Camouflage at 30, which gives you 5-seconds of near-perfect stealth and 30% increased movespeed. It also drops some of your aggro in combat, but it won't remove you from an aggro table like a Shadow's Force Cloak, so don't expect to reset that pack of mobs you just aggroed.
Levels 32-42: Planning for the future, then getting there
Here's our build at 42 - Force Exhaustion, finally. Force Exhaustion can be used to build stacks of Singularity, just like Force Stasis, but without that pesky need to be channeled. You'll want to be using it whenever you are able to ZL or Force Leap, so as to maximize your Force Sweep damage.
By now the number of new abilities has slowed down, but all the ones you get are at least kind of important. Cyclone Slash, your other AOE, comes along at 32 and (thanks to those initial two talent points) it also costs 1 focus. Still, Force Sweeping, then spamming Cyclone Slash rarely ever seems more efficient than just single-target-attacking everything after a Sweep. It does, however, look pretty cool.
Opportune Strike is available at 34, which is sort of like a weaker Pommel Strike that you can only use on slowed or immobilized targets. Given that many of a Sentinel's abilities don't actually do this, you probably won't find a use for it until 42, when you can use it on Force Exhausted opponents. Free damage is free damage.
Guarded By the Force is another fantastic ability you get at 38, but it comes with two caveats:
Don't put it somewhere where you'll accidentally hit it all the time and get your fool self killed, not that I know anything about that, and
You have five seconds to kill whatever it was that made you hit this button, so do not waste them.
Finally, you get Awe at 42, in which you look so awesome that you confound everything around you for 6 seconds. Once per minute.
Levels 43-50: The home stretch
Here's the build as you round the corner into 50. I personally suggest hitting up Defensive Forms first, for that additional 2% damage resistance, then filling out either Dual Wield Mastery or Focused Slash based on preference. Remember, once you have Focused Slash, your Shii-Cho Zen will cause your Slashes to give you 1 free Focus per Slash. You also get your last 3 abilities during these levels, and each one is fantastic.
The first, at 44, is Inspiration. This is, basically, Bloodlust. It requires 30 Centering, much like Zen. If you're in an Ops group, save this until your op leader calls for it.
The second, at 46, is Dispatch. I'd have to check, but I think Jedi Knights take longer to get their "execute" ability than anyone else. At any rate, enjoy using this - it's got a focus cost of 3, cooldown of six seconds, and a range of 10 meters, and is the only way Sentinels will ever throw a lightsaber besides Crippling Throw.
The last, at 50, is Valorous Call. Instant 30 stacks of Centering, usable every 3 minutes. Not a whole lot more to say about it.
And that's that
Now you're level 50! Go respec into something else, or keep being Focus if you like. Maybe you'd like to light things on fire, or pummel everything into submission. Or maybe you just like being ... Focused.
In which case, that's okay too.