How to lose or gain weight.

Let's say for example that you want to lose body weight or you want to gain weight, which is called bulking in the bodybuilding community. What this means is training then if you want to grow you consume more calories (food) to provide enough fuel for the body to repair and grow. Or vise versa if you want to lean down you consume less calories than you need to sustain your present body weight.
When trying to burn body fat, one thing that people need to understand is, that you can not spot burn body fat. The body metabolises fat overall, with the preference in regards to which region of the body to burn the fat off being purely genetic. (Distribution of body fat is purely genetic). To lose stomach fat, you have to lose weight overall. To lose fat, the body has to go in to a caloric deficit (you eat less than you burn through daily activity).

Let's say for example that your daily maintenance caloric input is 2000 calories. (If your weight does not change then 2000 calories is your maintenance). To change your bodily composition subtract about 200 calories from this daily intake to make a total of 1800 calories per day. To balance out the deficit, the body will burn fat for energy and weight will go down. Or vise versa, you add 200 calories to your daily intake thus making it 2200 calories per day. Over time the body will gain weight as the muscle will be repaired, get stronger and grow in size and with a surplus of energy being stored as fat for later.