I'm also using a Yamaha MT120. You are the first person on this forum that has mentioned that recorder. Pretty cool little machine.
I convert to CD using this approach:
http://www.sticksite.com/tape2cd/
Audacity is super easy to use, and it's free.
If your computer has a "line in" jack, you can use the internal sound card in the computer to convert to digital. All you will need to buy is the cord with the 1/8" mini jack on one end and the stereo RCA on the other end.
I, however, am using a laptop that only has a "mic" input, so I bought an imic (external sound card) that accepts a line level input, and plugs into the computer through a USB port. It's nothing fancy, but it's probably better than the internal soundcard for the reasons mentioned here
http://www.griffintechnology.com/products/imic2/
That first web page explains the process, step by step. The only thing he doesn't discuss is the use of an external soundcard (imic in my case), but it is very simple, and the imic comes with directions.
Please realize that people spend lots of money to do this process, and I'm sure you can get a better sound with an expensive sound card. However, why not try this first? I'm totally satisfied with the results I'm getting using this minimalist, inexpensive approach. Very clean conversion.
Let me know if you need any tips.