You probably want to start with something like
Fruity Loops just to quickly get your ideas into a playable format. Then later when you're really serious and are willing to buy top quality, perhaps you could upgrade to professional scoring software like
Sibelius.
I use
Cakewalk Sonar for all my MIDI stuff, but that's more a sequencing program with very little to do with proper scoring. However, Cakewalk have a wide range of products so you might find something that suits your needs in the
Cakewalk Store (check out the menu on the left).