1) Create a food diary - write down everything you eat with calories. You need to track your intake accurately. Don't bother lying to yourself.
2) Increase your exercise. If this is a short term thing, cardio will be best as ChuckG says as it burns calories quicker. If it's more long term, then weights are essential as they build more muscle than carido and increase ongoing metabolic rate.
3) Find out your basic calorie requirements for a day - google that phrase and there are plenty of calculators out there.
4) Create a calorie deficit. That is, burn more calories than you consume so that your body has to use your fat stores. A pound of fat is roughly 3,500 (K)calories, so you need to be eating roughly 8,700 less calories in a week than you are burning to lose 2.5 pounds a week.
It is possible, but it's a very high target (unless it's your first 10 pounds you want to lose, which do come off easier).
Don't crash diet - ie eat less than 1,200 calories a day if you are woman.
It's medically foolish, and it won't work. Your body will not just take energy from fat, it will take it from your muscles, which it finds easier to convert to energy, and so you will actually be doing more harm than good.