When and if time comes for the Seal training you can reach out to a Navy Recruiter.
Navy SEALs are awesome and I had the pleasure to work with a few. Outside of the military there isn't classes or course work that can prepare you for that life. If you decide to do it while in basic training (Boot camp) You will have to be in excellent condition and be an outstanding swimmer. There is a minimal requirement for swimming, Push ups, Sit ups, Pull ups and a mile and a half run. The run will be done in uniform do combat boots is a must if you want to train for the run. Of course SEALs use weapons so I would suggest that if you are underage you get with someone of age and learn proper gun safety and basics. Good luck on your adventure and I hope that one day I can call you my shipmate.