Could help if you included more definitions, such as "great" and whether it has to be a great way or whether you want to be making great money. Also Risk/Reward. At 15 the kids at my school who had a lot of money usually were dealing drugs, for example, so risk/reward definitely plays into it. If you want a legal, moral, ethical way to make money, you should specify that. Otherwise you'll get answers like "hack a swiss bank," "cheat at poker," and "steal the other kids' lunch money."
Or it just depends on what you like to do. Maybe you're just looking for a great way to make money, but you don't really care about how much money you're making. If that's the case, you just need to figure out what you like to do. If you like to swim, you could don scuba gear and go diving in your local mall's wishing well. If you like to sing, you could go downtown and sing, holding out a hat with a sign behind you that says "Feed The Starving Children of the World."
Or you could be like other 15 year olds and mow lawns, wash cars, or work at McDonalds.
I recommend you figure out what you want to do, then figure out if you can make any money at it. If you can't, but you want to make money, you'll have to resort to something you don't necessarily want to do.
You want fries with that?