How much weight are you trying to lose? You should first talk to your parents and enlist their help to make sure there is healthy food available for meals and snacks. You should also ask to make a visit to your doctor to make sure you don't have any health issues that would cause weight gain or make weight loss extra difficult, such as under active thyroid.
Many times, we don't really need to lose weight, we simply need to tone our muscles to look slimmer, so you might only need to implement and exercise program.
As far as diet, the first thing you should do is eliminate junk food, such as candy, chips, pastries, sodas, etc from your diet. Make water your drink of choice. That's often enough to implement weight loss without further dieting.
I am not sure how 900 calories is healthy for you. At 900 calories you are putting your body into survival mode where you will lose weight but not safely. I have several kids around your age that I train on a regular basis and they diet but SAFELY.
Like you said in your post you do need to know where your daily calorie intake is.
You can actually Google it for the formula put your information in and then you will know where it is. OR you can spend the money walk on a treadmill breathe through a mask and blow some candles like I did and then find out that the test and Google is about 18 calories away from each other.
When you find this information out then adjust accordingly. For example if your calorie intake is 1800 then cut out 200-300 calories of plus exercise will slowly help you lose weight.
Hope that helps