There's a girl I like but I found out today that she has a boyfriend and after that I started having emotional distress and I'm seeming very depressed why

WHY? Because the girl you like has a boyfriend! What more do you need to figure out? We all get depressed when we miss the boat or get dumped or generally fail in love. It won't be the last time, and you are in the company of millions just like you.
Did you ever do anything that would give her clues that you like her, like smile and say hi extra happily, or compliment her, or ask her little questions about what she likes, or just shoot the breeze? If not, then feel free to regret it and kick yourself a little, but then get off your butt and go be nice to all the girls, all the people you know, because even the guys will have sisters or friends they can introduce you to. In fact - be nice to her, even though she has a boyfriend. You never know what the future holds.

There's a saying: Don't make decisions by default. That means keep your doors and options open, keep going, keep being nice, don't barge in but don't give up either.
An example of a decision by default is if you don't ask a girl out because she might say no!

Keep busy with school, family, friends, study, hobbies. Confide in a best friend. It will pass.

You are supposed to be disappointed when you can't have what you want, but you aren't supposed to dwell on it, and feel sorry for yourself. Life goes on, and you should too.