i did this very thing last night.
took the RCA outs from the tapedeck into my patchbay, which goes via d-sub connector into my apogee rosetta 800 converters, which connect via ADAT lightpipe into my digi002r, which is connected via firewire to my g5. using ProTools LE, i recorded at 48kHz / 24-bit to AIFF, mixed down with POWr dither to 44.1kHz / 16-bit AIFF, which i imported as-is into itunes, then converted to a 192kHz mp3.
so it's pretty much as cr2sh said. 'cept i've spent a few more bucks on gear.
you need an analog -> digital converter. your mac has one built in, but it's not very high quality. you also need recording software, like Peak. there's a bunch of others. AIFF is a lossless format, so i recommend that for the conversion. you can import that into itunes, then convert it to whatever lossy format (e.g. mp3, aac) you want.