I want to know if this is good enough?

Ok im 15 years old and i want a flat stomach and less of the fat that is on it already haha but i just started last week and daily i have been walking probably approximately a mile or so a day and i have been also trying to eat less crazy unhealthy foods and i have worked my way up to 125 sit ups a day, 50 in the morning and 75 in the evening, is this good enough? I plan on continuing increasing the number of my sit ups a day but if you dont think this is good enough pleasesay so! and also i don't want a freakish six pack that is way unrealistic that some guys would die for lol but if you dont think this is good enough please give me some comments on what else i can do to help me do this! thanks in advance