I have a crush, but my crush likes my best friend, and my crush already said that he likes my best friend, what should I do?

You should accept that this is the way life is. We don't control other people; only ourselves.
If you haven't told your crush that you like him, feel free to do so, but try to keep it light and with a laugh, such as 'darn, I had hoped you would go for me' on your way out the door, or 'I always seem to miss the boat with the guys I like.' If you can't do it lightly, don't do it at all.

Other crushes will come along, and eventually you will develop relationships the real way, over time, and by little ways of showing each other your feelings.

Step back or you could lose your 'crush' and your best friend. You can't make someone want you, especially whenever you know they are interested in someone else.

Find a new crush.

Accept the fact that your crush does not like you and move on.

Look for another crush that doesn't already have a crush on someone else.