It's definitely the easiest MMORPG ever to get into (that's why there are over 9 million subscribers of it, most of which probably never even heard of Blizzard before it was released I bet

)
There's 4 basic things that happen though.
1. Make a character, level it to 70.
2. Get into Player vs. Player battles OR
3. Enter dungeons with 4-24 other people against the AI.
4. Re-roll (basically means make a new character and level that to 70 instead)
There's things in-between such as finding ways to earn money, improving your character's looks/gear and playing the economy, most of which you can do 'solo'.
Also
this guy did a huge survey from Blizzard's
Armory that shows you the most popular Races/Classes. There's roughly a 60-40 split between the two factions (Alliance and Horde respectively) with a lot more people on PvP-oriented servers.
I'd advise going on a regular PvP server, they are more active, have more skilled players and very good in-game economies. You will get killed from time to time by bored level 70s when you are levelling but that's the risk of playing on a PvP server, members of each faction can attack everyone of the other faction 99% of the time.