Watch as many games as possible to know what a water boy does and doesn't get to do. Pay attention to where he sits on the bench, when he approaches players with water or whether the players come to him. Knowing how to act as a water boy will give you a leg up on the competition.
Contact your local NBA team to find out how they decide who gets to be a water boy. Many teams hold tryouts and others simply have raffles. Each NBA team has a different method of picking water boys. Learn all you can to increase your chance of getting that dream job.
Ask your parents and relatives if they have any connections with the NBA team. It doesn't have to be players or coaches; anyone who works in the office can be a great way to get an inside track to becoming a water boy.
Attend the tryout if that is how your team picks water boys. Ask in advance what skills they look for and practice those skills as often as you can.
Fill out any required paperwork or entry forms if the team holds a raffle to pick its water boys. Fill out all the material and submit your entry as soon as possible in order to sit courtside.