I am 17 and I go to the egym around 3 times a week and I do mostly cardio but I do lift weights... now the problem I am having is losing my stomach fat... I do situps on the pilates balls and ill be sore but I don't see any results I feel like it's a waist of time, and I haven't lost any weight I stay at a constant weight... im 5'5 and weight 138 lbs... what should I do??

How long have you been doing pilates and situps? Also, how many sit-ups a day. What kind of diet do you follow?

Are there personal trainers at the gym? They can really help you figure out an exercise program to target certain areas. Make sure to drink plenty of water and eat healthy and all. Sometimes you won't lose weight for a while on an exercise program because you are gaining muscle at the same time you are losing fat. So it takes a while to see those numbers dropping. For me, the stomach is my "problem area" ; I always lose weight there last. Hang in there!

Talk with the trainers of course, and see what else you can add to your program. and then review what your eating... is it healthy and natural foods, in the right portions, Cutting out fatty, fast and fried foods, as well as high sugar and high salty snacks, drink lots of water, and cut out all presweetened high fructose corn syrup drinks like soda, keep portions small. It takes time to lose body fat with exercise as well as a over all body exercise program.

Cardio... it's the only way. You can do sit-ups until you're blue in the face and they won't do a thing. You need to bring your body fat percentage down. This is why all the gimmicks in the world are just fads. Everyone gets discouraged after buying some sort of ab machine off TV because they never get the results they want.

I would suggest doing 1 hour of cardio 3 times per week minimum. You need to gradually work up to this level. Start doing 20 min per session and work your way up with 10 min at a time until you reach that 1 hour level. This may take weeks. Use a stair climbing machine or treadmill. I would also suggest doing this first thing in the morning to utilize fat stores, IF you can.

And of course as already suggested, eat clean, and be wise about you food choices. Quantity isn't so much a concern, it's more "quality" in the foods we choose. Good luck.

Trainers are expensive. But to help you out we need to know more about your physique. What is your weight, height, and if at all possible what is the length around your stomach. You can do allot to loss fat but a really good way is cardio and swimming. They are the top 2 in my books. I help allot of people loss weight in my line of work. Plus eating right is a big key as well.
Have a look at this site and try to do all three areas. It will help
6 Pack Abs Exercises - Abdominal Sit Ups For Six Pack Abs