What does it mean if a girl asks you for a high five?

This could mean a couple things! He might just be being friendly or, more likely, he was flirting. Don't just assume he was flirting though; look for more signs from him that he could be interested before either returning the flirting or making it clear you aren't interested. Nothing is more awkward than telling a guy that you aren't interested when he wasn't flirting! Enlist friends and sisters and try and figure this guy out from other signs. Good luck :)