How can i prove it?

GENUINE HELP NEEDED IF THIS SITE IS LEGIT. I need to know how to prove that some one i know is doing something on the internet and with my computer, the person in question keeps denying everything but i know different, i also am aware that others involved havent got the guts to tell me either, this is and has made me ill. If you can follow what im saying i would appreciate any help, thanks in anticipation