Well first your 19 and just finished highschool go to a college and get on their team then play and then go to summer circuit were you play and invited to a nba training camp and then hopefully dont get cut then get drafted hopefully
If your a good college basketball player (or on rare occasion high school), NBA teams will look at you and can draft you in the NBA draft (when all the NBA teams pick players from college or highschool).
This is tough to answer due to the fact that the question is so open and too general. Its very hard to get into the NBA if that's what your asking. Playing in highschool is fairly easy but can be tough. Transitioning to college is very tough and very few make the NBA draft.