Well, I don't know how far you want to get into the whole midi thing. But there's things like Noteworthy Composer, Melody/Harmony Assitant. That's if you're pretty handy with music theory and actually writing music in staves.

Or you could try Logic or Cubase (one I use). But then you really need to find some good sound modules (I use EastWest's software, because it sounds amazingly like the real thing). Though, they are expensive sort of things.

Hears an example I made with midi, using EastWest's symphonic orchestra sound module: Download Here.