Qualifications
First things first, you need to be a super strong swimmer. They don't call it water polo for nothing. You should work on your upper body strength in general—lift weights, swim a lot, and practice, practice, practice.
Play as much polo as possible. The more you play, the better you'll get. Shoot the ball. The more times you shoot the ball, the more chances you'll have at scoring.
Start by signing up for Club Polo as early as possible—they have Splashball leagues for grade-schoolers.
In high school, get on the water polo team if there is one. Otherwise, stick with club. Play hard. Win.
If you want to continue to play after high school, here are a few tips to help you along when picking a college to play polo. Keep in mind, even if you end up going to college for something completely different and don't play polo, you can help your high school coach out part time as an assistant coach. This might even lead to a full time coaching job.
Once you're in college, be as impressive as you can be, try to make a name for yourself, and most of all, have fun. The more you play, the better you'll play. It's as simple as that.