What is some good software for creating music?

I always daydream. I daydream everyday. And one important aspect of my daydreaming is music. I always, no matter where I am, random melodies that I make up myself come to my head. And I wish there would be a way to sort of, make 'em exist. So I was wondering if there are any software programs that specialize in music composition. I don't need anything too techie, just something simple is fine. It'd be great if it was free too, but I'm good with a free trial as well. And I'd really like it to be without the requirement of an instrument, so that all the instruments were present in the software. I hope this isn't too demanding or picky, I thank you in advance though.