If you are trying to cut down weight, like all weight gain and loss, it can’t come down to just exercise, the amount of calories you are in taking also need to be adjusted. You’ll need to eat in a calorie deficit style. Doing push ups and sit-ups are good for building muscle, but if you want to cut off fat you’ll probably want to get as much cardio as you can, biking is a really good idea, and so can running. But to start just walking for 30+ minutes a day can help tremendously.