Qualifications
From the SEAL website: "To become a SEAL in the Naval Special Warfare/Naval Special Operations (NSW/NSO) community, you must first go through what is widely considered to be the most physically and mentally demanding military training in existence. Then comes the tough part: the job of essentially taking on any situation or foe that the world has to offer" (source).
It's never too early to start thinking about your future, especially if your future is going to be this hardcore. You can check out the military's tips for preparing for SEAL training while you're still a teenager here.
The qualification training takes twenty-six weeks. If you pass, you're assigned to a SEAL team and the real fun begins. You can become a SEAL medic (a very important function in the unit), an officer, or a specialty team member. You have to have good eyesight, meet the minimum vocation aptitude scores, be a U.S. citizen, and pass physical requirements for divers.
This job used to have one other qualification: a Y chromosome. As of 2016, however, women can become Navy SEALs. The first female SEALs could be getting their assignments by March 2017 (source).