Well, first you need an image that you want to use as an avatar.
Once you've got that, if you need to resize it, or if you just want to cut out a portion of it, you can use some sort of photo-editing program to tinker with it however you see fit. If you don't have any of the big fancy stuff, you can just use Microsoft Paint, which should come built-in to your computer (although MS Paint tends to make very poor quality upon resizing). If you can get it, I recommend Animation Shop 3, as it's great for resizing images and can also be of aid should you ever need to do some screen cap-taking from a DVD for any reason.
Once you have your avatar image how you want it, upload it either to or . Then, take the URL address of the uploaded image, and place it in your profile in the part where it says "Link to an avatar off-site" or whatever it says there. Then hit the Submit button.
If you can't get any good image-resizing programs yourself, just go ahead and post whatever image you want to be made into an avatar and I can do it for you.