General Questions:
When I was really little, I remember playing Mary had a little lamb on a toy electric keyboard. I played around on a friends piano when I was about 6, and I started playing a keyboard regularly when I was about 7. I haven't stopped since. When I was about 12 I started making short films that required music. I composed and recorded some very simple (read: bad) musical score on my keyboard, but I was more interested in making films than composing music. Record with MIDI keyboard straight into a DAW. If I have no access to my keyboard, I will notate ideas on my laptop using MuseScore (an awesome, free music notation program).
Composers who are self-taught:
I never really had a music teacher because A) There wasn't much about music that I couldn't learn for free without a teacher or school, and B) The hardest part of composing is the creative process, and that's something that I don't think can be taught. Hard to say, cause I don't know what it's like to not be self-taught. Flexibility. You can spend more time on areas you think are more important. I actually don't know. I've often wondered what you learn in a composition class, cause how can you teach someone to create something? You might be able to learn the technical aspects of the craft, but not the most important part, which is the ability to generate musical ideas. I think if anything, it would be a benefit. It couldn't hurt, anyway.