http://www.youtube.com/watch?v=v0WY7YxbGsg&feature=player_embedded
Frederik Steinmetz gives an introduction to After Effects expressions at AE Tuts in the first of a series. This is beginner stuff so if you've zero experience this is a good place to start. Watch it here. Afterwards, if you just can't wait for part two, you can visit MotionScript.com for more information or check out Harry Frank's expressions training title over at Toolfarm.