I'm currently going through the process of writing my next tune. Been trying different scoring software and ended up using Sibelius which is actually an app used a lot in schools. It takes a lot of practice to start writing music quickly. I'm not totally convinced on the interface. An opensource application called MuseScore has a much easier input method but it was just too buggy unfortunately.
Next thing to get is a MIDI to USB cable so I can make note entry easier. (At least I hope). This week is getting rather anxious for me as I've got to play my first scrored music in public and to someone from the Royal School of Music. So I'm spending all this weekend writing up my second piece and practicing the first one... and biting nails of course..