Your first step is to figure out how many calories you are burning each day. There are a few formulas that have come out over the years that estimate how many calories you burn per day based on your sex, age, weight, height and physical activity. When using these formulas, keep in mind that the number you get gives you an estimate not a number that never changes day to day. It will fluctuate.
Safe weight loss is considered 1-2 pounds per week. While it is possible to lose more, it is extremely unhealthy to do so. So how many calories do you actually have to cut out of your diet? One pound of body fat equals about 3500 calories. If you want to lose one pound in a week, you need to cut 3500 calories out of your diet per week. While this might sound like a lot, per day it comes out to a 500 calorie deficit.
Your next step is to ask yourself how you want to get rid of any unwanted weight. You can cut 500 calories out of your diet per day, or increase the amount of calories you burn by 500 calories per day. A combination of these two methods is the best way to go.