Purchase a computer-based recording program like Soundforge Audio Studio if you do not already have one. These programs can be purchased from most musical supply stores that sell audio software, or online at Amazon.com, for less than $100. A program like Soundforge will allow you to record material from many sources, including a minidisc recorder or player, onto your computer without sacrificing quality.
Connect the audio output of your minidisc player or recorder to the audio input of your computer. The connection you use will be based on your equipment. Many professional-quality minidisc players have optical or coaxial inputs and outputs. If you have these outputs and your computer has the appropriate inputs, it is best to use these ports. However, most consumer-grade minidisc players only have standard RCA cable outputs, and most computers only have 6mm stereo inputs. If this is the case, you will need an RCA-to-6mm stereo input cable to connect the devices.
Open the program on your computer that you will be using to record the minidisc. If your computer recording program has a "monitor" mode, apply it. This will allow you to adjust the levels of the song before recording it into the computer.
To get the best audio quality, try and get the loudest segments of the song to record at a level as close to 0db as possible. Recording over 0db will cause distortion, but recording too far below 0db will cause your program material to seem weak upon playback. If you do not have monitor mode, you will have to record the song and adjust the level as you go. Once you have found the appropriate volume level, record the song for the final time.
Repeat this process for every song that you wish to turn into an MP3. After you record the song into your recording program, you must save the file. Save your songs in an appropriately labeled folder, and save the files in MP3 format. They can now be transferred to iTunes or another MP3 player, or burned onto a CD-R.