Feeling slightly lost leveling thief.
Use a short bow, spam 2, and detonate the bomb before they hit the ground. If done correctly you can stack around 12-15 bleeds on multiple targets. Run in circles while doing this and you can solo pretty much everything.
For the spec, go with Crit as the base, it is a good trait tree to build into mainly beacuse it has trait slots that are beneficial to all builds. You however cannot do much for your traits yet though, only 10 points into each tree limits you quite a bit right now.
There is no “Best” build for a Thief, there are builds that work better than others for specific reasons and there are also many that people tend to favor, such as a D/D Death Blossom playstyle. So for the time being, try out all the weapon sets to see which one(s) you mainly favor and then build for that thereafter. Also keep in mind that once you hit level 40 and buy the book, you will have a full respec from it as well.
80 Thief 80 Engineer 0 Necromancer
80 Elementalist 80 Mesmer
I did most of my leveling as dagger/pistol with a focus on critical strikes. blackpowder is great for survivability against normal mobs, and can combo with heartseeker for a stealth, which you can then follow with a backstab. Once you feel your damage is good take some points in acrobatics and grab signet of shadows in your utility skillbar. These will help you get around the map much quicker, and the 5 point acrobatics trait will help you get away from things you don’t want to fight. Once you’ve got all of this down just work on completely clearing level-appropriate maps, and do your personal story quests when you feel you can handle them, as they give a lot of xp. You can also pick up crafting professions if you have the materials to get 10 levels per profession over the course of 0-400.