Don't get nervous on offense. When you get the ball, don't pass it right away. Take your dribble,
Playing varsity basketball in high school is not quite an impossible task. Some people are "locks" to make it, while others are on the bubble. Since I've had some experience as the latter, I'll share my lessons with you on how to make the team. You may never be a star, but you probably have a lot of passion to play and be a part of a team. Here's how you can do it.
Whenever possible, make an extra effort and make sure the coaches see it. Don't show up to practice on time, show up early. Dive for any loose ball, and always hustle up and down the court. You may not be the most talented player, but if you leave it all on the floor, coaches appreciate it.
Make friends with the other players. Coaches don't want someone who can't get along with the rest of the team. If they feel you'll disturb the team harmony or make things awkward, they'll be less likely to pick you. Spend time with other players off the court so it's easier to play with them on the court. They'll also be more likely to pass you the ball and help you out.
Initiate conversation with the coaches. Ask them what you could've done better on the last play or scrimmage. Show an eagerness to be a student of the game. I guarantee most players won't take this extra effort and will settle for learning only when the coach yells at them for doing something wrong. Ask specific question so the coaches can give you a real answer.
Since high school basketball coaches tend to be more conservative, defense, communication, and hustle are more important than your ability to jack up the threes. When you get on defense, make sure you harass and shadow your man relentlessly. Never ever ever ever jump at pump fakes or shot attempts, just stick your arms up in the air. Use your body to bump your man and spread your arms out, never allowing him to drive or shoot. Coaches like the defense; trust me.
Don't get nervous on offense. When you get the ball, don't pass it right away. Take your dribble, and keep it. Keeping your dribble lets you back away and reset. Call for picks and move around with the ball. Show the coaches you are confident and know how to make things happen. Take the ball to the hoop and break the defense down. Be an initiator.
Ultimately, you have to put in the necessary work to succeed. In the offseason you have to lift weights, run, and practice your shooting and lay-ups quite a bit. You can't cheat this part.