The avatar that the game provides can only give a general idea of what your character is like. It's up to you to fill in the details like height and weight, how attractive she is, her socioeconomic class, her family background, and her hobbies/interests. To help with the realism of other players being able to "see" things about your character such as an emaciated state, scarring, a rash, a missing eye, or what have you, you may want to try roleplay add-ons such as MyRoleplay or Immersion RP, which are used by a great many DPers.

Here is a basic outline of the bare minimum you should know about your character:

For World of Warcraft races, you can find a guide at the bottom of each race page that addresses these nine aspects of your character.

Go back to Step 2: Choose A Class

Proceed to Step 4: Flesh It Out