Learn the proper tackling technique. When tackling someone, get in the breakdown position, with your knees bent, eyes up and shoulder pads down. Take a step and drive your opposite shoulder into the chest of your opponent, while putting your face on the ball, shooting your hands through and grabbing the player's jersey. Don't ever duck your head and lead with the tip of your helmet when tackling.
Hit tackling dummies before engaging a real opponent. Hitting something that doesn't hit back makes it much easier to learn proper technique, while strengthening your confidence of tackling.
Practice all initial tackling drills at half speed, making sure you are in close proximity to the person you are tackling, helping to reduce the impact of each hit.
Practice tackling with players of the same size and aggression levels at first to build your confidence gradually and reduce the chance of injury while you learn the proper technique.
Study and watch videos of good tacklers in action to understand the techniques they are employing.