First, you track the calories in what you eat each day over the course of about 2 weeks to determine what your baseline daily caloric intake is. If your weight is generally steady, then you are burning equal to what you consume. Then, simply reduce it by 300 calories per day and you will start losing about 2.5-3 lbs per month.
Since you burn less as you get smaller, after about 40 lbs loss you will need to reduce it by another 300 calories per day to keep that difference in intake vs. burn.
The scale will tell you if you are maintaining enough differential. You should be consistently between 2-4 lbs per month lost. If you are at only 1 lb per month lost for 2 months in a row, you need to reduce the calories a bit more.
I can estimate your current burn rate at about 3200 calories per day based on your weight.