You cannot play defense if you don’t know how to tackle. You always need to be ready as a ball carrier approaches.
The first thing you need to do is to turn your body to be square with the ball carrier. Bring yourself into a balanced, athletic position by chopping your feet quickly so you stay agile. With your knees bent and arms spread wide, you’re more easily able to step left or right based on the movement of the ball carrier.
Once you clearly see which direction the ball carrier is moving, you need to step through them to make the tackle. Step with your head and chest up and shoot your hips through the ball carrier’s body while wrapping your arms around their body. Keep your feet moving to drive the opponent backward and maintain leverage.
Practice tackling regularly using a stand-up dummy or in live situations. Make sure your head is up and you are using proper fundamentals.