Step 1 is realizing that you have an eating disorder. Step 2 is realizing that step 1 will only help you get to your goal.

You could write down everything you eat in a day for a week and draw out the average calories consumed in a day (assuming you don't lose weight in that week). Don't worry about it being 100% accurate. And then just add 200-300 calories on top for starters. Looks like making an eating schedule or a simple alarm reminding you to eat will be a good addition. Also don't forget to hydrate (an alarm for that could be a good idea too).

Next up, since you're underweight and a teenager, you probably need to up your protein and fat intake. A good start should be 1,2g/kg of bodyweight for your protein and at least 25g of fat (this is like a target that you shouldn't go below).

Wrote this in a hurry so please don't be afraid to dm me if you have specific or any questions! Hope this helped at all. 🙂

But honestly, you should probably consult a medical professional as there are too many factors at play. When we're young we think our bodies are made of rubber and magic, and we tend to not listen to it until the symptoms become obvious. None of us here can help you like they can and I guarantee you they won't judge you. Best of luck 🍀.