My canon rebel xsi is not letting me take a picture on auto focus

It is possible you are trying to focus on somethind that is either completly white or completely black. It has trouble focusing in the dark as well. make sure your auto focus point (The red dots in your view finder is in the centre. you can move the red dot with the button on top of your camera that looks like 4 squares. simply look through your viewfinder push the shutter button down half way and the red dot should flash. if it isn't in the centre then move it with the button on top (4 squares) and try again until you get it in the centre.