6 ft 1 here.
Doing situps is stupid. Gaining muscle definition behind a fat stomach doesn't do **** for you. You need to do cardio workout for 30 minutes a day at a minimum of 3 times per week. That means get your heart rate up high, and keep it there without a break, for 30 minutes. Generally the easiest way to do this is not by running, but by power walking around town. Power walking gets your heart rate up, and it's easy to maintain. Some people think that by sprinting for 10 minutes they are exercising, they are wrong. It's all about keeping the heartrate high. Your body loses weight proportionally, so you can't just work your ab muscles to flatten your stomach, it doesn't work that way. It's a tiny workout and if it sheds any weight at all, it will be over the entire body.
Secondly, start lifting weights. The more body muscle you have, the more calories your body uses during the course of the day. Don't eat more, eat the same amount, and if you gain muscle, you will start to lose fat. The best way to do this is to start doing heavily weighted squats. The legs are the biggest muscle in the body, and if you turn them into huge masses of muscle, you will lose a lot of fat across the body. Also focus of course on the chest, triceps, shoulders, and biceps. Don't worry so much on defining your muscles right now, just worry about gaining muscle mass quickly and doing a lot of cardio.
Again, remember that running 3 miles in 20 minutes is not as good as power walking 1 mile in 35 minutes.
