Entering the Game
All races start out the game in whats called a starting area. Each race has its own starting area where you do low level quests and get a general feel for the game. Your default user interface will look something like this. (I will lable the different things on it)
Most people hate the default UI and opt to use addons to rearrange it. I will cover addons later, but basically you can change the size and position of everything on your screen. Move the map around, change its shape, add borders, move your action bars, add new bars, move and change your portrait, etc.
Here are some examples of user created interfaces
Example One