MIDI files(Musical Instrument Digital Interface) Two things you need are below.
1.some level of knowledge of music(you have to program the notes in. Compose/arrange/put everything together)
2. A program you can do it in.
Here are some. I use
Cakewalk Express personally. It was $30 but I've gotten tons of use out of it so it was worth it. You can probably download it online somewhere or a demo at least. Hope this helps somewhat. Let me know if anyone has anymore questions or anything, I could try and answer them.
Or someone else will know more.
Zack