If you are overly nervous or excited during a game, you are likely not to perform at your best. For example, you will likely miss shots you normally make at practice and turn the ball over with passes that go astray. First of all, you should understand that being overly excited is something that most players experience, particularly those who are just beginning to play competitive basketball.
Second, there are things that you can do to help:
Understand that experience will help you to become less nervous; that is, the more you play the less nervous you will become;
Do not worry or think about whether you will play well or badly. Just concentrate on playing--know your job, and do your job. For example, when shooting a free throw, concentrate on the shot, not on whether or not you will make it;
Do not worry or think about what your teammates, coach, parents, or friends think about your play. Doing so is a distraction. Concentrate only on the game. Lose yourself in it and just play.
Do not hurry. When executing a play option, make sure that you execute each step of that play option in the proper sequence. For example, when ending a dribble, make sure that you stop in balance before you execute a pass or a shot.