The Block Tackle
A good video demonstration of how to do a block tackle and not risk hurting your knee. In a block tackle you use the inside of your goot to win the ball.
If you go in hard and your leg is out too wide then you can injury your knee - most likely the MCL (Medial Collateral Ligament). So it's important to have your body over the ball, keep your toe inward, and not open up your leg too much. Take a look.