I was army but some of it applies no matter what service.
If I were you, I wouldn't worry too much about shooting. You need to start running, doing PT. In the army I think the requirement was 70% or some such on the old PT test but if you were infantry you pretty much were in the 90's or maxing it out anyway. You'll get to shoot when you get there and unless you're shooting NM courses there's not much you can really learn on a range yet. IF you can shoot NM courses, ideally with an AR and iron sights, that'd be okay. Some ranges have youth programs and can help with that.
I think the marines tell you what job you get, don't they?
If you're too smart, the army will try to put you in MI or some other MOS. I had to go 3 times before they'd give me what I wanted. Tried to intimidate me into signing up for what they needed. Whatever you do, DO NOT sign the form at MEPS until you get whatever it is you want (provided that's possible).
For marines, they like fresh HS graduates. Anything other than that and you only get a slot if they need it.
Take trig or calculus if you can. Differential and integral calculus is nice to know and will give you a chance to get the trig down better. If you can take classes at a local CC, that's even better (and they do a better job at teaching higher maths).
Did I say run yet? Yeah, run. A lot.
Keep your record clean. Don't piss hot at MEPS!
Talk to a recruiter now and see what they can do for you. You may be able to get a temporary military ID so you can use base facilities if there's one close to you. You could also get rank before entering (up to E4 in army, not sure about marines) if you passed the PT test prior to going in, if you learned how to march and some other details, if you have college.
The army will put you closer in touch with your goals of going to a sniper school. You also will get a shot at SF, guaranteed. I think you can get that in your contract too. I know you can get airborne and a shot at the ranger batt. straight out of basic (and you can get that in your contract too). My biggest military regret was not going SFAS ASAP. If you go ranger or SF, you may get a chance at other sniper schools including SOTIC. You might also get to go to a foreign or another branch of service's shooting schools.
With parents permission, you can join the army at 17, not sure about marines.
If you can knock out you associates degree before joining, you may be able to finish college while in the military (you could easily do this in 1.5 years unless you start while in HS, then maybe a year or less depending on how many classes you take and how much you can knock out in HS). Then you can apply for OCS at some point when you're done kicking doors and shit, wanna settle down some but still wanna be a badass. You'll get paid more too. I know a guy that did just this.
You should also do a lot of running starting now. Speed and endurance. Run until you puke. Or dry heave. Then hands on head and walk or shuffle the rest of the way back. If you're not puking, you're not putting in all the effort. Don't forget to drink a lot of water too.
When watching TV, do as many pushups and situps as you can during every commercial, on top of what you normally do. Swimming is good too.
Good luck to you!