It is important to understand that you generate your power on a tackle with the big muscles in your legs, especially the leg on the side of your body of the shoulder that you are going to make contact with for the tackle.
As you near the ball carrier, you want to shorten your stride, spread your feet a little wider and bend your knees. At this point, your back should be straight and your head up with your eyes wide open. Decide which shoulder you are going to use to make the tackle. Take a short power step at the ball carrier with the foot on that side of your body; explode up and through the mid-section of the ball carrier and make contact with your shoulder pad as your helmet slides to the side of the ball. If you are tackling at an angle, you want to slide your head in front of the ball carrier.
From here, you need to pound both arms around the ball carrier, grab his jersey in both hands, lift and drive him back.
However, many times in a game you will only be able to reach out and grab the jersey of the ball carrier. But you should try to use good form in tackling whenever it is possible, and never use your helmet to make contact on a tackle.