LOTS OF SIT-UPS!!! I am a young girl with an 8-pack. And literally I got them fast!!! I didn't have any fat to get rid of so I went straight to getting muscle. But if you have a lot of fat get rid of that first perhaps by running a lot and then gain muscle by doing a lot of sit-ups.
I would say not to do sit-ups; as they use primarily muscles in your lower back, and for all the action your abs work very little. Proper crunches are better, but I recommend the 'Iron Butterfly'. This is where you lay flat on your back with your legs up (off the ground 4-5 inches) and then do a crunch while pulling your knees toward you and closing your legs (so they are not extended). When you get into this 'crunched' form, you have your arms put out so you can grab your legs when you get them in. Another exercise is the crunch, and the crunch where you do the exact same thing as a normal crunch except have your legs flat (so to exercise your lower abs). And of course the plank is far better than regular crunches in the action of 'strengthening' your abdominal muscles. (Which allows for more, and better bulk building exercises