In order to get started you'll need a PC or Mac computer.

You'll also need to connect some sort of music synthesizer through a MIDI port or USB connection so you can play notes and they can be sent back to the synth, modified.   Or you could use a separate MIDI input controller, like a keyboard, and then use a separate synth module for output. 

In most cases a MIDI or USB driver will need to be installed using the disk that came with your USB keyboard, or MIDI interface.   Older PC systems may have a built-in MIDI interface connection using an adapter cable.