Me and my friends have been arguing over this for moths now. I think that it is a fruit but she thinks that is is a berry. Please tell me that I am right...

That would be a FRUIT ;)

banana - definition of banana by the Free Online Dictionary, Thesaurus and Encyclopedia. (http://www.thefreedictionary.com/banana)
This should also help What is a Berry (http://www.wisegeek.com/what-are-berries.htm)

Sorry but according to wikipedia the banana is a "epigynous berries or false berries", "Berries which develop from an inferior ovary" "as opposed to true berries which develop from a superior ovary"
However you are also right because berries ARE fruit, as apposed to a vegetable. So according to wikipedia you are both right Bananna's are a fruit in the sub class of berry. .