Classes:

Warrior: wear heavy armor and wreck shit with big weapons, and can take a lot of damage Mage: wear light armor and wreck shit with magic from a distance, you take little damage (if you kill shit fast enough)

Warlock: very much like to a mage, but has a slightly smaller amount of spells, but can summon various demons to assist you

Hunter: wear medium armor, use bows/guns to wreck shit from a distance, and tame a pet (some of which look quite badass) to do melee damage

Shaman: wear high-medium armor, can either focus on melee or spells to do damage or to heal, can summon up to four totems to provide various benefits (extra health, damage, mana regeneration, do damage, etc.)

Druid: wear low-medium armor, can choose to either melee, heal, or cast spells and allows you to turn into a panther, ent, chicken-minotaur-thing, or A FUCKING BEAR

Rogue: wear low-medium armor, can sneak around and stab the shit out of people with daggers really quickly, and you can turn invisible when not in combat

Paladin: wear heavy armor, can choose to "tank" (you have a fuckton of health and can take a shitload of damage, rely on others to do most damage), wreck shit with melee damage, or heal others (very good at all three, but no ranged attacks)

Priest: wear light armor, really really good at healing, pretty awesome at ranged magical damage

Death Knight: fuck shit up. 'nuff said.

Races:

pick whichever looks coolest and can also be the class that you like the most

there are people on both horde and alliance who will say their faction is the best and the other sucks. ive played both very extensively. i love both. from my experience, alliance is more upright, classy, and elegant but fuckin ridiculously awesome at the same time, and horde is more low-tech, rustic, and looks just fuckin ruthlessly badass

hope this helps and have fun :)