If you know anything about digital music files and processing, this is next to impossible. The reason is that MP3 (usually converted to WAV) is the actual waveform that make up a sound, where as MID (MIDI) is just the 'note' values. The more frequencies and harmonics, the more difficult this becomes. As the previous poster noted, it would work best with 'monophonic' (only one instrument/sound/voice) sounds.
However, if you do want to try this, the best programs out there are commercial, such as Roland's V-Vocal (found in Cakewalk's Sonar Producer software) and Celemony's Melodyne, but they cost alot of money, and typically work best with monophonic audio. Another, much cheaper option is to try Magix Music Maker Elastic Audio function. Otherwise, you'll have to have somebody try to figure it out by ear and transcribe it into MIDI for you.