Sit-ups and other abs work are great, but don’t count on them to reduce weight.
The only thing to do is eat less and/or exercise more.
I think the “diet” distinction bopils down to, whatever you do to lose weight, it has to involve an attitude/lifestyle change to permit you to keep it off. You have to change the way you think about eating, or the way you think about exercising.
One tip, look for ways you can exercise a little bit as a part of your regular daily activities. Walk or ride your bike somewhere you frequently go instead of driving. Use the stairs instead of the escalators/elevators for anything less than 3 flights. Promise yourself that if you have the damn TV on, you’ll be either on the exercise bike, doing sit=ups and pushups, or at least stretching out. That way, if you don’t feel like exercising, you won’t turn on the tube. Win/win!
With eating, try to figure out habits you can either eliminate, reduce, or substitute. If you usually have 2 scoops of ice cream, have only one. Try carrots instead of chips. Instead of soda, drink water.
Yeah, that’s another big one. Drink tons of water, all day long.
Final point, remember, the weight is only a number. You probably consider yourself 15# overweight because you are carrying more fat than you wish to. It may well be that with proper exercise you could carry an extra 15# of muscle, and look and feel pretty damn good. And the great thing about piling on muscle is, that bigger machine needs more fuel, so you can eat, and eat, and eat…
Good luck. Keep us posted.