It's scary how young some frequent flyers are... I'm young but 15 is really young. I mean, you can't even get a credit card for free miles.

You actually need to provide more information if you're going to get accurate help. What kind of fares do you (by you, I'm assuming your parents) usually buy? Where are you based more, LON or NYC? What do you want from your FFP? Any favoured carriers you have? Any carriers you want to avoid? How many miles do you estimate flying? Is domestic lounge access important? (although keep in mind you'll likely be barred from entering the lounge alone)

Do you actually want to go to SVO or just MOW in general? (If it's the former I think a psychological examination is in order for you as well) SVO is mainly a ST hub, so unless you want to fly DL/SU specifically there is no reason to choose that as your only airport option in MOW. Actually, just by looking at the cities you want to fly to, AA/BA would probably be the best programmes for you. You'll have the benefit of an AA hub at JFK, BA hub at LHR and an S7 hub at DME. Between those 3, you can get pretty much anywhere in the world worth mentioning except for Southeast Asia and Oceania. DME also has connecting flights to Tajikistan, something I don't think SVO has.

Both AA and BA FFPs have their merits and disadvantages, no way to tell without more information which is more suitable.