There are only a few ways in which audio can be recorded to a computer. The two main ways are through a microphone or through the auxillary input port. It is as easy as it sounds.
To record through the microphone, simply connect your microphone to the computer and start your recording software. This software could be Sound Recorder found on the Windows Operating System. To record through the auxillary input port, connect the output of the playback device to the computer.