Just found out today myself how to change your in game looks of your character! its really simple and would be fun to see more then just normal people running around!
First go to http://www.minecraftskins.com/
browse TONS of skins for you to pick from. ( already made you can of course make your own )
Once you have found the one you want to use simple click the download or upload option to the right of the image.
If you are downloading it simply download somewhere to your desktop. I
If uploading it you wont need step 2. Make sure you are logged into the minecraft.net site.
Once you have saved the skin you want to use go to http://www.minecraft.net/
Log in with your minecraft name
Go to profile at the tophttp://www.minecraft.net/skin/
There you simply choose the skin file you saved from minecraftskins.com
Once it is uploaded CLOSE FULLY out of minecraft.. re open and login.
Enjoy your new skin!!