How do you accurately find it? Start with A guess of how much you should be eating. Then, You eat, accurately tracking everything you take in - food, drinks, oils, supplements, everything. You track your weight. Every day, weigh yourself in the morning, after going to the bathroom, with minimal clothing (underwear at most). Tracks the trend of how your weight changes, compared to your average calories. It takes a 3500 calorie deficit to lose one pound a week, which is 500 calories a day. If you're losing one pound per week, you do 1.0 lb * 3500 calories/lb / 7 days to find your daily deficit per day. Change the weight number to reflect your actual weight loss.

This is a pain to do manually, because your weight fluctuates a lot due to water, food, whatever. Normally I'd recommend the app called Macrofactor, which does all this math for you, but it costs $6/mo over a year, which you may not be able to do at 16. There's a spreadsheet on r/fitness that you can use instead, which does similar things.

https://www.reddit.com/r/Fitness/comments/4mhvpn/adaptive_tdee_tracking_spreadsheet_v3_rescue/

If you're not losing weight as at the rate you want to, adjust your average caloric intake based on your actual deficit and your desired deficit - more calories to slow it down, fewer calories to speed it up.