How do I prove to my professor that I didn't copy pasta another kid's work?

Well, if his professor is like most of them out there, I'm gonna guess the guy isn't there 90% of the time to help the students out (Even during their designated office hours). Asking your fellow classmates for help / working together is basically the only thing you can do most of the time. That's what it's been like for me in college so far. >:|