Focus points on Canon 550

Hi
I just got an Canon EOS 550D.
Im still new and learning, so Im using the "green square" function,
and Im using autofocus.

But the points in the display decides by them self, where the focus are gonna be.
How do I set the focus to be in the middele allways ?

You need to be in P, Av, Tv, or M mode to make that change. Press the zoom button (upper right on the back of the camera) and either rotate the main dial or select the points with the cross keys. The set button toggles between all points and center point.