Personally, I believe you can exercise all you want; if you don't control your diet it's almost a waste of time. Exercise is always good because it strengthens the heart, lungs and creates better circulation, etc.
But a poor diet will always set you back. When you go shopping, buy foods that are low in fat, cholesterol and calories and try to cut out or reduce your intake of pizza, ice cream, candies, etc. Also, try replacing donuts, burgers with fruits. I guarantee, you will lose weight gradually
but you have to discipline yourself. Don't starve yourself, remember it takes time to gain weight and it will take time to lose it.