to loose weight,you have to cosume less calories than you expend. weight gain is generally aslow process where people find themselves consuming morecalories than their bodies need to perform daily function
in activity combined with consuming too many calories is the main culprit of weight gain. both can be controlled by you . you make the choises to do or do not. your previous decisions in years past has brought you to where you are today. it's all cumulative.
determine your current average daily calorie intake(also known as you maintainance calorie intake). you need areference point of where you are currently so you can determine where you need to go (regarding calorie intake).
if you have determined that your average daily intake (without losing weight or gaining weight) is 2500 calorie , you may want to start by consuming 500 calories less, or 2000 calories