I think it would help if you learned how to properly converse with people. It could maybe be difficult since you'll need to do that with high schoolers but you should try it.
There are more you could try reading. I would also recommend observing people's conversations as well to know how to go about them in practice. It's simple small-talk they engage in and just a chain of respond-and-speak.
As for how to start them. You sort of have to "break-in" to social circles and such. I would recommend just talking with people who have similar situations and after-school acitivites as you.