My iTunes won't backup my iPhone 5. An error message always comes up!

I just got this iPhone 5 and I backed up all my info from the computer from my iPhone 3G. Well I try to back up my phone once a week and i just noticed the phone hasn't been backed up by iTunes. A message always comes up saying "iTunes could not backup iPhone because an error occurred."

I also tried to just back using iCloud but it says the same message. What do I do???