I agree with boldstar.
You may see more than one focus point light up and think that all points will be in focus. But this is easier said than done.
Remember that to focus, in very simple terms, the internal lens distance to the sensor in the camera body is adjusted. If you have a number of focus points that are varying distances from the lens, whatever the lens does, there will be points not sharp in focus.
Try to set you camera for a single focus point - the center is a good one.
Then lay the center focus point on part of your subject and take the photo. Now check if this point is in focus.
Remember to have a high enough shutter speed to eliminate camera shake or movement as a cause of soft shots.