Hey man, I'm 15 too. I'm joining the military too. Not sure exactly what yet, but definitely something in combat arms. Really looking into Air Force Pararescue. Make sure you're dead set on the Navy SEALs. It's no joke. BUDs is a 6 month haze fest. Statistics say you won't make it. Beat the statistics!
I would suggest running 2-5 miles 5 times a week, mixing up length and intensity. Don't overdue it at first, or you'll injure yourself. By the way, you can get kicked out of BUDs for getting injured too bad, so be careful and smart if you make it there. Have a light day, medium day, intense day, medium day, light day, etc., with the running. DO NOT carry weight....yet. When you get to be 17-18, then start rucksack marching about once a week with at least 45 pounds. Why wait till then? Because we're still growing, and this is the perfect time for you to stunt growth or develop shin and knee problems if you rucksack march regularly at our age. Trust me, that won't help out.
A little weight lifting is good, but you won't see any weight in BUDs. Focus on push ups and overhand pull ups. Vary push up positions. Be a push up king. You''ll do thousands, if not more, in BUDs. Every time you or your boat crew mess up, you have to drop and do twenty(if you really want to, and are self- disciplined, drop yourself every time you mess up at something). Push yourself, and strive to improve your reps every time you workout. If you quit during your own workout, you will likely fail BUDs workouts miserably. Keep that in mind.
Get mentally prepared. Have fun though. We're 15, and this will likely be the best part of our lives before we make it in the real world. But, focus in school and stay in shape. Research BUDs and Navy SEAL history and news. Why do good in school? Because the ASVAB requirements for SEALs are the highest in the Navy, besides the Nuke rating. And they're pretty much Einsteins. Operating is a smart man's game. Also, you'll have to learn dive physics and demolitions, etc., at BUDs.
Good luck man. Know what you're getting into. It's a deadly proffesion.