Click on the icon at the top of the forum page that looks like a gear, and says "My Settings" when you hover over it. On the User Settings screen that comes up, expand out the User Settings section and you will see the field for entering your signature and a field for browsing out to select a new avatar.
"Sadly then I knew the answer. All her life she was a dancer, but no one ever played the song she knew." - The Residents