If the opponent is running towards you, and you are just standing on your position, tilt your body in such a way that if he/she runs from one side, you can find it easy to run with him/her. Always try to show the opponent the outside of your weaker foot. This means that never let them run inside or they will make it hard for you to tackle them down. Simply show them the outer side. Take them away from the goal. You can do this by covering the inner side of the pitch with your body, which will automatically make the other player run outside from the penalty area, making it easy for you to tackle him/her down.
Once the player has started to run, tackling the ball becomes relatively easy. Start running with your opponent and wait for him/her to make a single mistake. Every player has the tendency to knock the ball longer than it is required. As soon as he/she does that, either hook the ball with your stronger foot, or simply knock the ball out of the player’s reach. Most players would knock the ball out of play.
In case you are just standing and the other player is standing as well, tilt your body again and do not give him/her the space to get past you from your weaker foot. As soon as he/she runs towards your stronger foot, simply knock the ball. Do not give too much time to the player or he/she will try to come up with an easier way out. In case you have been beaten, a sliding tackle can always come in handy.
A sliding tackle is quite easy to perform. You simply have to dive in towards the ball. Make sure that you don’t have your studs faced up or else you can get a red card.