Tackling Without Pads?

Alright I know how to tackle but tackling different kinds of people proves to be difficult. Im 15 5"10 150lbs. One kid is like 5"9 107lbs. He is very easy to tackle but getting to him is hard. We play on a pretty big field for 5v5. I usally cover him and when he gets it or runs the ball he is very difficult to get to. His cuts are mad crazy and it gets tiring trying to get him. Another kid is 5"10 220lbs. Just getting my arms around him is difficult. I can get him from the back or the side but when hes running right at you he just plows through you. I try to go for the legs sometimes but when i go for them he just runs over me. The last type of person that is difficult is that hes 18, 6"1 210 lbs and very strong. He likes to run through tackles and lower his shoulder. When he gets into the open field no one rlly goes for him beacuse he is too tough to take down. I am asking you guys for help if any of you have any technics or tips on how to tackle these kinds of people i would appreciate it.