Scratch

You need a program that can create animated .GIF files, like Flash or Blender. If you google "animated gif maker" or "animated gif avatars", you'll find some sites that let you make moving images out of a series of still ones. And of course, there are loads of animated gifs sitting around on the internet. If you find one you like, save it to your computer (right-click it and select Save Picture As, Save Target As, or something similar) and then upload it back to Scratch on your profile page.

@Shiguy101-You could make your own topic, (like you did) but don't ask it on someone else's topic. And, it's want and icon, but you're a new scratcher, people don't care that much...