EMusic is one the major competitors to the iTunes Store for delivering legal song downloads to iPod users. eMusic distinguishes itself from iTunes by offering a flat-fee monthly plan, rather than a per-song price. To learn more about eMusic read my review of the service.
Once you've done that, if you decide to become a member or just try eMusic's free trial, here's a step-by-step guide to downloading music there.
Start by going to eMusic.com and choose "start your free trial".
Starting with your free trial will sign you up for free eMusic MP3 downloads and to continue the service after your free trial ends. (You can always cancel your membership after your trial ends, if you like.)
Enter your account and payment information, and choose your membership plan, depending on how many songs you want to download each month and how much you want to pay.
Whereas the iTunes Music Store charges you for each song you download, with eMusic you pay a flat fee each month to download a fixed number of songs.