Im in love with my best friend but she has a boyfriend and I don't know what to do

You leave her alone. She's taken. When she's single then you can tell her how you feel, and pursue her. Until then she's off limits.

Ya same situation happened to me a while back. Now everyone can't give two sh*ts about me and I've been really depressed lately. But don't spoil a friendship with going out

You do nothing. She has a boyfriend.

I don't think you should tell her yet. However I believe that if you really truly love her she'll learn hat she loves you too one day and you won't have to tell her that you love her whilst she's taken.
However, if that doesn't help change your mind and you still want to tell her then I'll answer your question
Do it in a sweet way, but try not to do it in a way that would scare her. Write something beautiful or say something beautifu that will make her realise that you're not messing with her. Buy her some flowers or some chocolates Remember, if she shoots you down, DO NOT keep trying until she's available. She may think you are stalking her or something and could become extremely frightnened which would never give you a chance...
I hope I helped :)