To lose weight, you need to create a calorie deficit.
This simply means that you need to eat fewer calories than you burn.
A pound of body fat equates to approximately 3500 calories. Therefore to lose a pound you need to have a 3500 calories deficit (be it by calorie restriction or increasing the energy burned, through exercise, etc.).
So....If you are currently eating 2500 calories a day, or if you calculate that to be the amount required to maintain your current weight, you could create a calorie deficit of 20% by eating 500 fewer calories a day. Or you could burn 375 more calories a day, or combine eating less with burning more to achieve the 500 calorie deficit.
2500 x .20 = 500
Over time you will learn how your body reacts to the changes you make, and you will make the necessary adjustments to reach your goals.