First you need to find an avatar that you like. This is simply a small image that you wish to represent you on the forum.
Have a look in www.heathersanimations.com
for a start, or try a Google search on 'avatars'.
The image needs to be hosted somewhere so that you can link to it. Then it's simply a case of posting the URL in your profile setting. However, linking to someone else's web site in this way is not recommended because it is considered bandwidth theft. Ideally, you should upload the image to your own personal webspace and link from there. If that is not possible you could email the image to us and we can host it on your behalf.
Let's assume you have posted a suitable image on your own webspace. If you right-click on your chosen image and select 'Properties' you will get the URL of your image. Copy this URL and then click on the 'Profile' button at the top of the page. Scroll down to the Avatar control panel and paste the URL into 'Link to off-site Avatar' field and then hit the submit button.
Your image should be no wider than 125 pixels (and preferably no taller) and the resolution should be set to 72 dpi.
If all this sounds complicated, it isn't really. Give it a go. If you get stuck there are plenty of people here ready to help you.