The other posters are right that you should do cardio as well, but there are exercises that specifically target your abdominal muscles.
If you are doing full situps, then you won't be training your abdominal muscles properly. When you get past the point of a crunch, you stop using your abdominal muscles and instead your body is pulling itself upwards mostly by your back, shoulders and neck.
There is a video here explaining crunches:
https://www.mayoclinic.com/health/abdominal-crunch/mm00725
There are a few important points they don't explain in the video.
* Keep looking forward, do not collapse your neck to your chest at any point.
* by putting your hand on your legs, this is the easiest from of crunches
* By crossing your arms and resting your hand on your shoulder, this is the medium difficulty crunch
* By placing your arm by your head, fingertips touching your ears, this is the most difficult form of crunches
*Once you are comfortable with all of these, move onto crunches on a swiss ball to help core stability too.
You can also do bicycle crunches:
https://www.ehow.com/video_2351933_bicycle-crunch-exercises-abs.html
Leg Raises:
Lie flat on your back
Raise your legs above you to they are straight
Lower halfway to the floor, keeping them straight
Then back up again, repeat x 12, rest for 30secs then x 12 again
Plank
This is amazing for core strength, and abdominal muscles.
https://z.about.com/d/exercise/1/5/q/A/plank1.jpg
1. Lie face down on mat resting on the forearms, palms flat on the floor.
2. Push off the floor, raising up onto toes and resting on the elbows.
3. Keep your back flat, in a straight line from head to heels.
4. Tilt your pelvis and contract your abdominals to prevent your rear end from sticking up in the air.
5. Hold for 20 to 60 seconds, lower and repeat for 3-5 reps.
