Aight the first thing to remember when tackling someone is that your strength comes from your legs. Keep a good, strong base with knees bent and left foot forward so that you can charge with your stronger leg (vice versa if left-footed). Head down, aim to ram your shoulders into his midsection. At the same time, hook your arm around his legs and try to lift him from the ground with your momentum so that you can take him down to the floor.