Go to gravatar.com and sign up with the same email address you're using for this site. You'll get a confirmation email.
Click the URL in it, choose username and password, and you're taken to a form where you can choose to a) upload a picture from your computer (your old avatar, for example, if you have it), b) copy the address of a picture on the web, or c) choose one of their avatars.
On the next page you can crop what part of the picture you want to use as your avatar, and then you have to confirm your choice. After that, it's a question of time before the same avatar appears by your posts here too.