I get many versions of this type of question. Gaining or losing weight is largely about calorie control.
As explained in an earlier post, body weight is determined by the balance of energy in (food eaten) and energy out (energy expenditure).
Therefore, you would alter your energy intake/expenditure depending on what your goals are. Ken has programmed a very good calculator which assesses your goals and needs and gives a calorie range to consume here. It also gives possible workout routines.
Try not to change your routine too drastically. Going from eating 3000 calories to 2000 calories a day is very disruptive on both body and mind. It is doubtful that such a change will lead to a long term lifestyle change.
So how do I change my diet to lead to a long term improvement?
A good number seems to be about 500 calories. Change your calorie consumption up or down by about 500 calories. If, after 3-5 weeks, the results you attain are not what you were hoping for, increase or decrease it further.