As a chap, you need approximately 2,500 calories a day for living. You use less energy sleeping (everything slows down) than you do while awake. That 2,500 is for a single 24-hour period including around 8 hours' sleep.
One pound of fat stores approximately 3,500 calories. Thus if you cut down to a daily intake of 1,500 calories, you will lose roughly 1lb per week, which is a healthy rate at which to lose weight.
If you cut your calorie intake too low, your body will believe there is a famine, and will work to retain as much body fat as possible. It does this by, essentially, ingesting everything else instead. This means you'll get your energy from absorbing your own muscle mass while retaining fat. So try not to do that
If you cut down to over 2,500 calories a day you will not lose weight, you will simply gain it at a vastly reduced rate.