Once you've created an account or logged in, go to the top right where you'll see your username:
After clicking on to your name and going to your user profile page, click edit profile:
From there, you can change your picture by selecting Change avatar, add a description about yourself, and fill in as much or as little information as you care to. When finished click the Save Proflle link.
Leave a reply if you have any questions or something doesn't work for you.