If you install Nokia PC Suite (http://www.nokia.com/pcsuite), you can simply drag & drop (copy) MP3 tracks (or other formats such as AAC, etc.) to the phone's memory card and start listening with the phone's built-in music player.
You can also use the PC Suite supplied Nokia Audio Manager to rip your own CDs, but any other ripper/jukebox will do just as well.
You can also use an USB memory card reader on your PC and copy audio tracks directly to the card that way, and then put the card in the phone.
Get a big-enough memory card for your music. There are different size cards available; 64MB, 128MB, 256MB, 512MB (MB = megabytes ), and at least ATP has announced 1GB DV RS-MMC/MMCmobile cards. Note the type: it has to be either DV RS-MMC (Dual-Voltage Reduced-Size MultiMedia Card) or MMCmobile type cards. No others will work.
If an MP3 track averages, say 5MB, a 64MB card will hold ~12 tracks, a 128MB card ~24, a 256MB card ~48, a 512MB card ~98 tracks.
If an MP3 track averages 3MB, a 64MB card holds ~20 tracks, a 128MB card ~40, 256MB ~80, 512MB ~160.
If one CD has about 16-18 tracks, a 64MB card will hold about 1 CD worth of music, a 128MB card 2, a 256MB card 4 and a 512MB card 8.
This, of course, assuming that there are no installed programs or photos or other data also taking up memory card space.
And with the N70 you can also listen to FM radio broadcast channels.