If you want to control your aperture settings you will need to work in AV mode (see top of your camera). I have always worked this way as it gives complete control over the depth of field. Best read up on it though until you become comfortable working that way. Your manual will explain it, but there are loads of tutorials on the web. See this Youtube tutorial for instance:
http://www.youtube.com/watch?v=ZB7g7Qhpw6g