When you start Minecraft, your avatar comes preloaded with a default look or "skin". Thanks to the open and maker nature of MC, these skins can be easily changed anytime you wish. This is a quick run down on one way you can change your avatar's skin:

1. Go to the "Skindex" http://www.minecraftskins.com/  and browse through the hundreds of user created skins.

2. Download a couple that you like and save them to your computer (you'll need to find them later.) 

3. Go to http://minecraft.net and log into your account. *Make sure you are NOT currently logged into the Minecraft game!*