A most-basic iTunes question

I have a Mac Mini with my music on an external drive. After reading about improvements in sound after doing a clean install of Snow Leopard, I decided to go ahead and give it a try. Trouble is, I can't recall how to tell iTunes, which is now pristine, that the tunes on my external drive are the ones I want in my llibrary.
I know it's really simple, but ...
Thanks for helping me past this brain-block.
-- Howard