I would take your 3 shaman (simple), and do WG and AV. Being in a larger group helps give you some time to figure out what you're doing without always being focused (which is going to happen when you're boxing in general).

Use your pve gear - you'll get rolled with it on, but you'll also be dishing out the pain. I guess maybe it depends on your pve gear, but I assume you have pretty good gear or you wouldn't be asking. If it's marginal, yeah, get the blue crafted stuff.

Best thing to prepare is to understand you will probably need an entirely different set of keybinds and macros, in addition to a different mindset, obviously. Understand that you have pay careful attention to staying alive. While that's true in pve as well, while pve'ing you generally know exactly what is happening and what is going to happen because you've seen it all before. pvp is dynamica, and you never know when that rogue will jump you. Your power goes down exponentially as you lose toons, so I focus very hard on not letting anyone die. Plus it save a lot of headache about ressing a toon clear across the bg.

So yeah, just jump in a give is a whirl, you'll love it.