is to LOCK your auto focus. This means that you depress the shutter button halfway, (two beeps will sound) so that you can see in the viewfinder EXACTLY what is and is not in focus. Make sure that the subject is in focus, and if necessary, repeat the half-press. Then, without lifting your finger, press it down the rest of the way to take the shot, all the while keeping your subject in the frame. Are you sure you are locking the focus every time? This is absolutely essential.
Burst mode and RDS are good for later on, after you master focus.