An Avatar is a little picture that is below your name and to put it there you have to modify your profile, also the size of the picture matters Mine is 80 x 83 pixels I don't know how many pixels are allowed but this seems to be about right for this forum.
I do have an avatar as you can see from this post with my name on it and piano keys and music. I found the picture on the internet and just added my name to it after rezing it.