well, I don't know how much you already know but I'll go over as much of the basics as I can... but keep in mind, in some ways I am still learning myself, lol
First you need a picture. I know you can use an actual picture but it can't be too big. Someone else would have to tell you how to resize it. So I recommend getting a premade avatar. They are all over the net but I recommend you start here:
http://www.thefuselage.com/Gallery/ Unfortunately, it hasnt been working the last few days. But hopefully it will be fixed soon. You could also google "Lost Avatars", "Lost Icons" or some such variation.
Then save it to your desktop. I have a mac laptop so I click and drag but you do whatever works for you

Now, open your profile from the button at top of the page; (in the bar under the header) Then look along the left side column and click on "Forum Profile Information" Now, it
says you can upload your own picture, but it doesnt actually work.
You need to open an account with
Photobucket or another file hosting site. I only have experience with photobucket. When you have that account you upload that avatar, and under that picture it will give you a list of 4 links. Click on the one that says "direct link" (which will automatically copy it for you) and then return to your Forum Profile Information page.
(almost done, I promise)

Click the button next to "I have my own pic" and paste the link from photobucket into that box. Go down to the bottom of the page and click save profile.
So if you already knew half of that, I'm sorry, and if you didnt and it sounds too confusing I still recommend you give it a shot. Once you get the hang of it, playing around with avatars can be a lot of fun. (I have 103 avatars in my photobucket account! I'm a little obsessed)

Good Luck! and of course, if something doesnt make sense, ask again!