Ditch the crisps, upto 3 bags a day?? and have a healthier meal in the evenings. Pasta is fine but burgers & chips? I expect you don't really need telling on those points, its not rocket science. Just eat better.
As for cheap exercise can you leave the house for a walk, run or bike ride?? Or are you a single parent and have to stay indoors of an evening? Anything to burn calories.