Hey, welcome to the thief class!
PvP/WvW
Nerovergil’s advice for WvW is good, but I would advise looking at the build Yishis runs since it’s a D/D build that, personally, I think works better. His build is here:
http://intothemists.com/calc/?build=-c;4RwkZ0J5R-Vd0;9;5TT9;225B35A0;3JNV4-F04m9fgm9fgm-FTF6-6-N_schVlWYXv21475230B-2c;5kV05NV05;9;9;9;9;86-6U
And you can see a video where he explains his gameplay here:
http://www.youtube.com/watch?v=NEEYhovtwBo
If you use D/D in PvP, be sure to pick up the trait in the Shadow Arts tree that grants blindness on stealth since this will contribute to shutting down your enemies while also allowing you to land stomps against downed opponents that try to interrupt you.
Also, for anyone new to the thief class, I would highly recommend watching the below video, as it gives you a lot of little tips about the thief class for tricks that will make a big difference in your gameplay:
http://www.youtube.com/watch?v=eUmrfBr1R5s
Additionally, there are a lot of good shortcuts that you can take in sPvP maps with the short bow when you use Infiltrator’s Arrow. I would highly recommend experimenting around on PvP maps with this and/or watching some videos that show how these work.
http://www.youtube.com/results?search_query=gw2+infiltrator%27s+arrow&sm=3
Another tip that I didn’t see in the tip video above is for getting strong burst when using D/D: When stealthed, cast Basilisk venom and move behind your enemy. Wait until your stealth is just about to wear out and cast Cloak and Dagger. If you time it right, you should land the attack after the stealth wears off on its own. This will prevent you from getting the revealed debuff and will allow you to immediately backstab.
PvE
The builds above are ok for PvE as well, but generally, you can go for more of a glass cannon build that focuses on damage since it’s easier to dodge predictable NPC attacks. Surviving on a thief is harder than probably any other class, so it may not be best to go for a full glass cannon build imediately, but ideally, a 25/30/0/0/15 (http://gw2skills.net/editor/?fYAQJAo4EBnLOpcORs9Yn6kn+HIrODa2SA) build with berserker gear (power/precision/crit damage) will allow you to contribute maximum damage to your group.
As for which skills are useful, here’s a list of some of the best skills for thieves in PvE and PvP along with a brief explanation of why they are good:
Shadowstep – Stunbreakers are crucial in PvP and this is one of the best. It not only can break stun twice (once when you port away and once when you port back), but it will also give you awesome mobility and more options when stomping enemies (see vid linked earlier). The condition cleanse is an added bonus.
Shadow Refuge – This skill ends up in most PvE and PvP builds because it’s incredibly useful for so many situations. You can prevent allies from getting stomped by stealthing them and then rezzing them, you can hide a group of allies past enemies, you can stealth yourself right before you die, then port away with your downed #2 skill to rez in stealth, you can get 2 stealth attacks in PvE with the right timing, and you can also use it to stealth for a stomp.
Signet of Agility – The overall versatility of this ability makes it pretty amazing, particularly in PvE. It can save you by cleansing conditions or simply giving you extra dodges and when you don’t need this, it will increase your dps by boosting your precision.
Blinding Powder – It’s a simple, on demand, instant stealth that costs no initiative and can be cast while stunned. This will also stealth allies that are very close to you.
Signet of Shadows – If you don’t have Runes of the Traveler, then this is a great way to get around quickly. However, it really shines while you’re leveling up since it just lets you get around faster.
Infiltrator’s Signet – The extra initiative passive is decent, but the active ability is a stunbreaker that will also port you to your target. As mentioned in the video, this is great for landing stomps against other thieves who port away while you’re in the middle of your stomp animation.
Shadow Trap – The long range on this makes it great for moving around in sPvP, such as when you need to get back to your team to help out in a fight. Outside of this, it has a few situations where it can be helpful, such as the Swamp fractal in Fractals of the Mists.
Assassin’s Signet – When you don’t need the utility of other skills, this is a great way to simply boost your dps. It can also be activated for some extra burst damage.
Anyways, I know this is a lot for one reply, but hopefully it helps. =)
