Look under your current avatar - for nre players that would be a quotation bubble. Directly beneath it you should see an option to change your avatar. The selection isn't exactly expansive, but you should be able to find one that suits you!
Hi, I'd like to know if and how I can get an avatar here, in the forums?
If you missed the opportunity to choose one when you made your account, you can always change it. On any post you've commented in, like this one, simply select the picture below your name and a list of available avatars should appear.