This may be something other than what you are looking for, but the main thing is going to be to practice. PvE can help to some extent, but you really need to just get in there and start playing. That's the only real way.
I understand you're possibly nervous about not being able to contribute for your first couple times in PvP or WvW, but that's really just how it's going to be. You will get much better much faster by actually playing. People probably won't even notice, and even if they do, it's just part of the game.
Beyond that, the biggest piece of advice I can give is to be familiar with your skills. You will tend to get nervous in PvP, moreso than in pressure situations in PvE. Make sure you are familiar with your CC effects -- which skills they're on, how they work, how long between uses, that sort of thing. Again, gauging how effective each skill is will only be able to happen through actual experience.
Other than making sure you know how to use all of your skills (with focus on CC), you'll want to practice kiting -- again, this is best done against other players, but you can try kiting mobs around as practice.
Familiarity with other classes/skillsets will help -- I suppose you could try playing with all sorts of weapons/classes/races so you will know ahead of time exactly what to expect from every opponent. Jumping right in will still be a faster way to gain this knowledge than trying to prepare without actually PvPing.
Best Answer
If you check the wiki entry on Structured PvP, you'll find that:
This update removed separate PvP items and introduced the wardrobe instead. All you need to do in PvP is to equip any PvE weapon with your character and while you're in any PvP match, it will have the stats of its level 80 exotic equivalent. The sigils on it can be selected in the same PvP build panel (at top of screen while in Heart of the Mists) as your PvP traits and the amulet. See the wiki entry for PvP Build for more.