I have been a high school golf coach for 30 years. Many posts here are partly correct, but they do not go far enough. A solid coach has to know the game just like a football or basketball coach knows his game. Learn everything about teaching the fundamentals that you can.
Teach your players how to hold the club, how to swing it in all possible situations. How to handle sand, hilly lies, long putts, short putts, chips, pitches.
Team them the rules and course etiquette. Teach them the best strategic way to play each course, in relation to their own type of ability. Make them realize that they must continue to compete, even on a bad day, because they are playing for the team, not just for themselves.
Being a good high school coach takes a lot of work. i put in 25-30 hours a week, but it is worth it when you see the players improve, and the team get better.