Viewed as a system, the body has inputs of air and food, and has outputs of energy, air and waste products. The difference between input mass and output mass is weight lost or gained.
This difference is under your control. To take control, you need to understand your adjusted basal metabolic rate (ABMR), which is the amount of number of kcalories required to operate your body at its current size, age, and level of activity.
Estimate your ABMR according to the Mifflin/St. Jeo formula:
If m is your weight in kilograms, h is your height in centimeters, and a is your age in years:
ABMR = (9.99m + 6.25h - 4.92a + c) * x
Where c = 5 if male and c = -161 if female; and where 1.2 <= x <= 1.9, and is a factor used to correct for level of activity (1.2 is sedentary, 1.9 for highly active).
This result is an estimation of your minimum kcalorie intake per day to maintain weight.
Next, calculate the amount to reduce your ABMR needs in order to lose weight:
R = (3500kcal/lb * t) / 7
Where t is the weight in pounds per week you wish to lose. (There are roughly 3500 kcalories in one pound of fat.
Subtract R from ABMR to obtain your per day calorie target. Stay near (or under) this target consistently per day.
Recalculate your calorie target as weight is lost or as exercise is added to your weekly routines.
Weigh in daily and track with a moving average for a more accurate view of your weight loss trends.