Hi Kelly! I'll try to answer your question :)
First find an avatar that you like and save it on your computer. It can be any picture that is about 100x100 pixels in size. Some people at pf make them to share with others and they can be found in this thread:
http://pictureframe.greatboard.com/viewtopic.php?t=51
In order to use your avatar here, you need to upload it from your computer to a photo-hosting website. You can get a free account at photobucket.com, that's the one most people here use.
When you have the picture at photobucket, click on "profile" at the top of the page ^^^ and scroll down to the avatar control panel. Copy and paste the URL under the photo at photobucket into the box next to "link to off-site avatar". Then your avatar should show up here!
Don't be afraid to ask more questions, I'm pretty slow at computers myself so I had to ask too at first. Oh, and aim is AOL instant messenger :wink: