Go to the Dashboard to edit your profile.
If you want to add or change your avatar you have two options. You can create a Gravatar (a globaly recognized avatar) by going to Gravatar. Gravatars are recognized by many sites including this one and facebook. Just make sure you associate the e-mail address used here with your Gravatar.
You can also go to your profile page and upload an avatar (bottom of the page). If you have both a local avatar and a Gravatar, the local avatar will take precedence.