Start improvising and record yourself doing it. I guarentee you that within a few days, you'll have come up with a something you can't get out of your head. Compose your first composition around that! (It doesn't have to be a good composition, remember. You will improve by continuing to compose)
I think that is the easiest way to compose.
I've never heard of a professional composer who hasn't had at least a bachelors level education in music.
No. Paul McCartney most likely can read sheet music...
Easiest, but certainly not the most mature or worthwhile. It will provide you will dated and uninteresting results. I recommend approaching like this:3) Develop your aural skills to a high level. Nothing is worse than hearing something in your head and not knowing what it is.