Horde or alliance. On that note that's all on you man. If you wanna Flag areas with blue and Put horde Into the grave go alliance. if you wanna smear the blood of your foes on your flag go horde. Not much of a difference just different races and racials.
If you are new to wow I would say wrath because of its development. but since you are also new to wow and playing on a pvp realm beware of something called ganking. Its were People allot higher then you Kill you Just because they can. So be weary of that.
Classes depending on what expansion you are playing on differ from power. In bc, they are not all balanced so some are op and some are weak as crap. If you want to heal You can go (Druid, Shaman, Priest, Or paladin) If you want to tank go (paladin warrior Or deathknight) Deathknight is available on wotlk not bc and you must have a character 55 or greater. and for dps well Any class can dps.
Do your research find a guild you want to join work torwards getting in that guild And learn. Also, be warned Wow can be a toxic place!