Depending on your inital strength and endurance, do 25 every chance you get. Then, when you feel confident keep doubling. 50 to 100 to 200 so forth.

Sit ups will build the muscle under the belly fat, the larger the muscles, the further out the belly fat get's pushed. You need weight loss, not crunches.

May I suggest that you do 25 - 30 sit ups TWICE a day .... Also; perhaps you should also power walk about a mile from your home and a mile back EVERY day ... it will assist in making your metabolism work better and you will see more weight come off .... Also; when you drink the water ; make sure it is COLD or Ice Water ... drink one 16 ounce glass BEFORE each meal at least .. and a glass between meals ... Try to get ONE salad at least per day ..NO dressings etc. Best of luck to you.

since we dont know your wieght and level of fitness this is tough. but if you are healthy enough for exercise and want to diet. i would start with 1500 calories due to the fact that you need alot of calories and vitamins when building muscule. you need to stretch real well and do a light arobic exercise to start. like some rowing or simple jumping jacks, squat thrusts, push ups, and jogging in place. then you want to do leg lifts and crunches. i would say sets 2 of 30 each the first week and double it every week for 4 weeks. you need to make sure to not hold your breath on the up motion. on the second week and a set of crunches that use the oblecs and do them side to side. when you are done make sure to drink water slowly and go for a short walk. you should do that 3 times a week. on days you dont do your work out you need to walk, jog, run, or bike. this will burn fat and allow the muscles to rest. you need a rest period to truely build muscle. and the arobic days will keep your heart and lungs healthy as well as burn fat. never push yourself harder than you can take. if it starts to hurt then stop. i dont mean pain as in working muscles i mean pains in your arms chest or neck. and never push a muscle to hard you can tear it and you wont build it cause it will spend its time trying to repair. all this is just opinion. you should get the advice of a doctor or trainer.