adding to the functions.php

hi there,
i was hoping someone could help me with something that i’m sure is very straightforward allthough i have searched and googled but can’t find the information i’m looking for.

basically i have recently changed themes and in my old theme was using a custom functions.php that a friend wrote for me to handle recent comments. my new theme allready has a functions.php so i need to add the info from the old one into the new one but don’t know where to put it. i tried just slapping it in the bottom but this killed my blog ??

i can post the contents of the files if that would help, or if anyone knows somewhere that details how the functions.php should be laid out and added to…

i would be very greatful for any help,
many thanks,
xxsxx