Do you shoot in Manual or Auto mode?

The problem you describe sounds like it might be related to the aperture size you are using, as that is what dictates depth-of-field (how much of your photo is in focus). There should be a way for you to manually change your aperture size on your camera. For shallow depth of field, you need a larger aperture, which corresponds to a smaller number.

Different lenses have different max/min aperture sizes, so it could be that your lens is the issue.