There are a couple of different types of Mic. The two most common are dynamic and condensers. Dynamics dont need power condensers do.
So an adaptor that converts an XLR (3 pronged mic plug) into 1/4 inch will be fine for a dynamic mic providing you run it into a Mic (not a Line) level input. Mics output LowZ signal which is very different to Line level
An easier option is to buy a USB condenser microphone. Blue and Sampson both make them. they have a built in pre amp, sound reasonable and will let you shoot directly into the computer.
MIDI is whey cool. it is not audio it is more flexible that that.
If your soundcard has a midi sound generator built into it you will be able to record midi data and then play it back through the sound card. You will also be able to change patches / instrument sounds as suits you. If your sound card is midi compatible you may be able to plug a midi device directly into it providing you have a cable with the correct ends.
Or you could spend a few bucks on a USB / midi adapter. they are not expensive.
Equivalently, you could run the headphones out into a line level input. This is not ideal as there is probably an impedance mismatch however with a little experimenting you should be able to record something worthwhile.
Good luck and have fun !