Focus. Fall into the FOTM Sweepfest.
You're still going to feel rather weak IMO til about 25.
Yeah pvpwise I'd say guardians/Juggs are very much among the late-bloomers, so if you feel weak or ineffective in the early goings don't worry about it-- it isn't you it's the nature of the class.
For all classes generally you can break leveling ranges into chunks of about 10 (i.e. two tiers) so looking at the guardian
Vigilance-- Very utility based with items like an extra sunder armor stack, longer immobilize duration on leap, etc. Unfortunately without a big toolbox this does not help very much.
Defense-- Actually a reasonable selection at this range due to the no cost blade storm (especially since you don't have many good focus building/saving options yet), nd focus saver of victory rush
Focus: While it doesn't become a FOTM spec until you start building singularity stacks, there's a lot of good low tier benefit with flat boosts to smash and master strike, crit boost on slash.
For this reason Defense is Viable, Focus is solid, and Vigilance lacking
Vigilance: Perks up a bit here. Shien form +unremitting start to give you some pvp value that make this actually a fairly solid tree at this level range.
Defense: This is where you start getting tanky, unfortunately it also becomes where your damage starts to fall behind the curve. Without some of the late level utility you'll really find yourself unhappy with this tree most likely in this range, having mediocre damage without enough utlity/itemization to feel tanky enough.
Focus:where this spec starts to become smashbomb. Zealous leap is a very handy gap closer and damage dealer in one while you start accessing your abilitiy to build singularities for big-sweeps at the tail end of this range.
Vigilance-- Holds up ok but not drastically more exciting then the previous echelon, the one advantage is that your growing emphasis on DoT help is much less likely to be cleansed sub 50.
Defense: HIlt strike is a quite nice utility aspect and there's some surviability help, but really at this point going this deep into the defense tree is basically a huttball-only item
Focus: If tier 3-4 was smash, this part adds the bomb. Shii-Cho Mastery and Focused resonance flesh it out from being a strong aoe spec to perhaps the strongest damage dealing spec in game right now.
Capstone (31 point talent and beyond)
Here Focus remains far and away the most effective with force exhaustion now giving you a much more on-demand ability to build singularity without relying on somewhat longer cooldowns. Beyond that it is just about fine-tuning items that make it more impressive.
That said at this point you also have enough talent points that Defense/Vigilance hybridizing becomes possible to serve as basically a tanky class with decent damage dealing. Neither spec is that impressive for pvp 31 points in, but when you can start mixing them together you have a good veriety of utlity, survivability, and enough damage to contribute.