Jeff, a great forum to bookmark is
vBulletin Articles A lot of it may be over your head at the moment, but there is a LOT of information in there that will help you out.
Most modifications have instructions in the thread or in a Read Me text file. Usually you would go to Products & Plugins > Product Manager and on the bottom it says Import Product. That is the easiest type. Some of them require Template Edits - Admin CP > Styles & Templates Make sure the modification you choose to install is for your version of vbulletin - don't pick a Mod in the vB3.0 section for your 3.6.8 board.
I would suggest starting with an 'easy' mod first to get the hang of it. Good luck and have fun!