I wouldn't worry about PvP until at least 35. While you can go now, the 10-19 and 20-29 brackets are filled with twinks. Twinks are people who spend hundreds of gold to get the best gear and enchantments on thier gear cause it makes them feel big to go and beat up people who don't have higher level toons to fund that kind of thing. Twink Rogues in the 10-19 bracket will pretty much one or two shot clothies.
As for pets, quick run down
Imp - Once you get Dark Pact(40+) he is a great Mana Battery. The rest of the time he is a great Party Pet for the Sta Buff.
Voidwalker - Primarily used for solo, he's your meat shield. Doesn't bring much to a party setup, and only real use in PvP is saccing him for a shield. Though IMO there's better PvP pets.
Succubus - Probably the highest damage standard pet(can get without talents) and also extremely useful for CC. Theres a great macro for using her CC on wowwiki.com
Felhunter - My persoanl favorite PvP pet. Helps see Rogues, and chews up Mages by preventing them from casting a school of magic.
Felguard - Must spec 41 points into Demo to get. He's like a cross between VW and Succy, a more offensive tank. Great pet if you like Demo spec and learn how to use him properly.
You get a few more DoTs if you go Affliction spec, namely Siphon Life and Unstable Affliction. Both are gained through talents in the Affliction tree.