They aren't "taken" until they're married.
What should I do if my crush has a girlfriend. I know that it's kinda wrong to like someone if they're taken but... I just don't know?
It is not wrong to like someone when they are in a relationship. That's perfectly normal and just shows that you are attracted to that person. It would be considered wrong however, to try and split them up. Simply wait for them to end their relationship, or move on.