Jerry,
First of all, you need some kind of recording software. I would suggest n-track, since it is simple, effective and cheap
FASoft Web Site .
When you have the recording software, record the minidisk track into a wave file. You can do it by simply plugging a 1/8"-1/8" cable from the MD's line output into the computer's line input.
Now you have the backing track, on which you can add you guitar and vocals.
You will need a microphone. While a cheap dynamic microphone may certainly be enough for your voice, yuo simply cannot get a decent accoustic guitar tone with one. Believe me, I've tried and keep trying
, so I guess you should get a general purpouse condenser, that should be enough for all your needs.
You will also need a mic pre-amp, in order to get the mic signal into your computer (The mic level is way too low, and need to be brought up to line level). You can often find mic/mic preamp combos that you can buy for about 200$ in the US.
Good luck!
Oren