You need to calculate your daily calorie needs. The calculation is based on your current weight, height, age and activity level. That will tell you how many calories you need to maintain current weight. Once you know that, you will need to decrease you calorie count by about 500/day, in order to lose weight. Apps can show you where you might be able to cut back, or where you need to eat more.
boz5886 is right. This App will help you stay on track. Do you have a particular weight goal in mind and also a date in which to accomplish this goal? Having a realistic and meaningful date will help you stay motivated. There is a plethora of websites you can go and checkout to see what your calorie intake should be for your height/weight/age/activity level. A typical calorie intake for someone such as yourself would need to intake about 1600 calories daily in order to loose approx 1 pound per week. Of course the proper macro-nutrient levels would have to met also besides just the calories, i.e. carbs, proteins and fats. good luck.
This app doesn't decide anything. It just helps you count the calories that you are eating. You have to decide if what you are eating is too many.
Keep an eye on your weight tracker, not just your calorie counter. That will tell you if your following your weight loss goal.