Eat fewer calories than your body burns up and you will lose weight. You can best accomplish this but eating less and burning more through exercise.
To set a target for yourself, try this. Calculate your Basal Metabolism rate by multiplying your weight by 10.9. That will yield your BM, the calories your body burns doing all the things it does automatically (heart beating, breathing, making new cells, processing food and waste, etc.).
For example, if you weigh 150 pounds, multiply that by 10.9 (women should then reduce the product by 10%) to get 1472.
If you limit your intake to this number of calories per day, you will lose weight. You'll lose it faster if you exercise.
As with anything, consult a physician to be sure this is OK for you.
Note: The actual calculation for Basal Metabolism is a multi-step process I have consolidated into one multiplication.