Get an exercise ball and start with doing five sets of ten sit ups, twice a day for a week and five sets of ten push ups on the ball. In the second week up yourself to ten sets of ten repetitions and you'll notice the difference in no time.
Lie on your back and lift your kness up to a ninety degree angle. Focus on your core by pulling everything in to stabilise your body. Then slowly, remembering to continue to breath throughout, lower one leg down on the floor and then back to the starting position. Repeat on the other leg. If you are doing this correctly you should only be able to manage a maximum of ten a time.
Sit upright at your computer and don't rest your back against the chair. Between your internet searches or whatever, take a deep breath and hold it for the count of say forty and then exhale.
While you're holding the breath your stomach should feel firm to the touch.
The best way I find to tone your tummy is by sucking in your stomach for at least fifteen minutess a day. This will also improve your posture which makes you look taller and thinner.
Find a ninety degree corner in the kitchen and with your arms on both straight sides and your bum in the corner lift yourself up and strech legs out in front of you. Your abs are doing most of the work and your arms are being worked as well.
Sit ups on a gym ball are supposed to be really good for you.
If you like your alcohol, try to drink vodka with diet coke or tonic water. These have less calories in that any other alcoholic drinks.
I'm fifteen and I've been doing yoga for a couple of months and it has made me lose some weight. I also cut out crisps and sweets and replaced them with fruit. I would suggest half an hour of Yoga, an hour of power walking and around forty sit ups a day.
Get on all fours; make sure your hands are under your shoulders and your knees under your hips. Keep your spine in a neutral position and don't flex it during the exercise. Exhale all the air from your lungs, relax your abdomen, but don't increase the arch in your lower back. Next, pull the belly button up and in toward the spine, without motion at the rib cage or pelvis. Hold for twenty seconds.