You say your stomach is flabby - do you mean it's still fatty or it's just not toned? If there's not much fat there it could just be that the muscle needs toning; the advice about situps should work as it will help tone the muscles and tighten things up. Since you're only 15 your body will do this a bit on it's own as well, if you give it a few months.
If your stomach is still fatty then I'd need to tell you that you can't target fat-loss in one area. I'd love to tell you that it was possible, but sadly it's against the rules of the universe. When your body burns fat it does it from all over. Because you're only 15, your body is at the point where it's going to start change shape anyway and become more curvy (NOT fat, just curvy. Like Beyonce
). It will naturally start collecting more fat on your bum, belly and boobs (hooray for boobs!), but this isn't necessarily EVIL fat, it's just WOMAN fat.
For the next couple of years the important thing for you isn't going to be losing fat so much as making sure you keep eating well (i.e. good food, not over-eating of sugar and junk) so that your body can grow healthily into the shape it's going to be, rather than it fighting you as you starve it and wear it out with too much exercise on not enough food. When you get to 18/19 that's a good chance to stop, see how your body has womanised, and then look at a new health and fitness plan to make the most of your new shape.
Don't we all! But being happy with how you look isn't always about trying to force your body into the shape of your dreams with a tiny tummy and massive breasts. It's about learning that you are the person that you are, and even if you never look any different that's totally ok. To say "I'm 83% happy, and that's pretty good!" Look around next time you go shopping and see how many people there are that look different from you, with funny noses or a couple of spots. Very few of them will make you go "Ugh! Gross!" and they certainly don't see you that way. We all have to learn this.