Purchase and install MIDI-enabled audio editing software such as Digidesign Pro Tools, Adobe Audition, Apple Logic, Sony Acid Music Studio or Ableton Live if you have not already done so. Try Garage Band if you are a Mac user, as it is a free utility that comes preinstalled on your computer through which you can perform MIDI functions.
Assign your computer keyboard as the utility's MIDI controller if necessary. Select your computer keyboard from the list of MIDI devices in your audio editing software's "Preferences" tab. Download Virtual MIDI Controller if your audio editor does not inherently support computer keyboard-as-MIDI controller function. You can use Virtual MIDI Controller to assign keys and act as a new MIDI controller that your audio editor can recognize.
Assign computer keyboard keys as desired. Open your audio editor or Virtual MIDI Controller's "Preferences" tab and set your computer keyboard keys to play specific notes. You may need to open a "MIDI" tab under or separate from the "Preferences" tab, as these default settings vary by program.
Push the assigned keys in appropriate musical order to arrange your music. Play the computer keyboard as you would a musical keyboard or touchpad in your audio editing, recording or production session.