What's the difference between a girlfriend and a Valentine?? I really need help on that question???

Someone can be both. A valentine is generally someone special to you specifically on St. Valentine's Day. It'd most likely be your girlfriend. She's still your girlfriend the rest of the year round but she's also your valentine on February 14th.

Someone could also be your valentine but not your girlfriend.

Valentine=Someone you like who may or may not be your girlfriend.
Girlfriend=Well, your girlfriend.