Talk to the school basketball coach at the beginning of the school year. He can tell you whether or not you will need to try out to make the team, or whether the team accepts any interested students. In either case, you will have to demonstrate your skills at one point simply to be placed on the varsity or junior varsity teams.
Show up for tryouts early. Warm up, making sure you are ready to show off your skills.
Examine the competition; try to identify their strengths and weaknesses. For example, if there aren't many good defenders in the group, you can focus on demonstrating your defensive skills during the tryout.
Demonstrate leadership, fair play and kindness during the tryouts. These qualities are all important in a high school team, and, even if you lose the point, you will impress the coach with your sportsmanship.
Follow up with the coach afterwards, building a personal relationship with him. Politely stress your interest in joining the team, as well as what you think you can contribute to the team, and then leave him to make his decision without pestering him.