How do I accept a student? How do I coach them, do I just play games and tell them there mistakes and tips or do I go to a room and talk with them or something?

If you want to coach, the first thing to do is to find someone lower-rated  who  needs some help with how they play  the game.  And begin playing games with them. Every student requires a personal approach to his or her needs. So this you will find out while playing with them. There is the "messages" box in the game where you can talk during the game. Or you can discuss the game with them after it is finished .