What should I do if I get tickets to meet a celebrity who has a girlfriend/boyfriend but I have a crush on them?

Be respectful of their privacy and their lives. They are simply people, no different than the people you meet everyday or go to school with. They just happen to be in the spotlight where millions of people they don't know, know them. It's really weird for many celebrities. When you meet them, just tell them how much you appreciate their work, whatever they do, and tell them how much it means to you. If you have a specific story to tell, tell that. They appreciate knowing that whatever they do affects people positively. Don't tell them you have a crush on them, just treat them like a normal human being.