Answer:
There really is only one way: you must be very talented, and you must be at least 19 years old (or have played one year in college).
The NBA is virtually impossible to make. The first step is being very talented and dominating in high school. Those select players get a scholarship to a college. Then, you must be dominant in college, and that will give you a shot to be taken early in the NBA draft. There are 60 selections in the draft, so you still have to be talented among the talented.