SEAL Teams seem very glamorous and for this reason, there are many young men who apply. Of course there are not many applicants with the fortitude to become SEALs, but still you will have to compete with all the other recruits who want to get into BUDs training as well. Once you get accepted into to BUDs there is lots of harsh physical training. If you are in good condition, you can succeed. But you have to have the attitude that you would rather die than quit. When you are cold and tired and haven't slept in 4 days, it is real easy to quit knowing that a hot shower and warm bed are only a few hundred yards away. You must put the idea of quitting out of your mind completely. That includes the idea of quitting trying to get into the Teams if you are not accepted in the first place. Keep at it.
Once you are in BUDs, there are lots of skills you have to learn aside from physical development. This means you will have to study a lot. If you hate school and do not have the discipline to study and learn a difficult subject, forget being a SEAL. It requires a lot more than just being big & strong. Start now by learning how to study. Learn how to love learning the things that will help you in your career. If that career is in the SEAL teams, know that whatever you are required to study and learn will help you achieve that goal.
Overall the best advice I can give you at your age is to start now to develop the right attitude that you will need to reach your goals. That is good health, a positive "can do" attitude, willingness to work as a team, and the willingness to study hard. Above all NEVER QUIT SHORT OF YOUR GOALS.