It is important to understand that weight is entirely a function of input and output. The input is the food you eat and the calories contained therein. The output is your energy output. To lose weight the output needs to be greater than the input. It is that simple. Do not believe any of the diet fads. If you are currently not gaining or losing weight then just burning 300 extra calories per week or eating/drinking 300 calories less per week (2 sodas for example or a small burger) WILL make you lose weight - in this case around 5 pounds of fat per year.
It's recommended that adults should try to be active every day and should complete at least 150 minutes of moderate aerobic exercise per week - this could include cycling or walking at a fast pace. Alternatively, you could complete 75 minutes of vigorous aerobic exercise, which could include running or a game of football. You could split this up into easily manageable 30-minute workouts over 5 days of the week.
In a way, moderate-intensity physical activity is that "magic pill" a lot of people are looking for, because the health benefits go beyond keeping your waistline trim: Not only can it reduce your risk of cancer, stroke, diabetes and heart attacks, but studies have shown that physical activity can significantly improve the moods of patients with major depressive disorders.