Most high school coaches look for multiple things. But they usually don't determine if they want you by what position you play (unless there are other people trying out for the same position). One thing they look for is speed. All coaches love players who are quick on their feet because they can beat players and get back faster. Another thing is skill. If you are speedy but not very skilled, you won't be the coach's first pick. Skill is the main thing that all players need. From controlled first touches to craftiness, skill is a key factor in what makes up a great player. The next thing coaches look for is cooperation/chemistry. If you're agile or talented but you can't work with others, it absoulutly drop your value.

Overall, if you have some of these traits (usually not all) then a coach will want you on their team. (Skill being the most important, then speed, and finally cooperation). If they see that you are working hard and you have good potential they may also want to pick you.