You need a USB card reader. You simply plug it into a free USB socket and put the card into the relevant slot. You can then copy (or 'drag & drop') any files on your PC onto it.
Card readers are cheap. I've seen them for sale in both Asda and Tesco Express at reasonable prices. They're also easy to find online:
http://www.ebuyer.com/product/161219
(I've even seen them, on a few occasions, in '99p' or '£1' shops).
You need to remember that some music files downloaded from iTunes (including all pre-2009 downloads) have DRM ('digital rights management'), so that they'll only play on an iPod (or within iTunes software); they can't be transferred to other devices. MP3 files downloaded from other sources should be free of DRM.
Chris