Great question! The amount of calories that you need every day depends on a number of variables such as: biological sex, age, height, current weight, level of physical activity and what your goal for your weight is (gain, maintain or lose). There are many tools online that can help you to determine about the right number of calories for you; one of them is the called the “Body Weight Planner” and you can plug in some information about yourself and receive an estimate of the number of calories you may need per day. Typically, an active male teenager needs between 2500-3000 calories per day.
As for understanding the number of calories in foods, there are a few ways to gain this information. If it is a packaged food, you can read the nutrition label to find out how many calories are in it. If it is a fresh food, such as a fruit or a vegetable, you can use an online tool such as the USDA supertracker to get an estimate of the number of calories. The supertracker also will estimate the number of calories in prepared foods. An example of when you may use this would be if you eat at restaurant and are curious about how many calories were in an item that you ordered. The supertracker will be able to give an estimate of about how many calories are in typically prepared items. With counting calories, it can be challenging to be accurate, so it’s better to consider these number as estimates.
If you have more questions about nutrition, it can be helpful to talk to a healthcare provider such as your doctor or a registered dietitian.