There's one program called "Anvil Studio" that allows you to create MIDI files - either put the notes in by dragging them up and down the stave, or by using the keyboard that's on the screen.

http://www.anvilstudio.com/

To learn to play the music, though, will take more than Anvil Studio. If you've got a Keyboard at home that you can play, then that helps. And you should be able to find online Piano Lessons/guides and sheet music to practice with.

It's not that hard - one hand is especially easy if you can read music and know which key is which note. Two hand gets a bit harder because you're reading two parts at once and therefore need to do two different things.