Write. Just write.
Don't worry about format, font size or whether you're doing it wrong. Don't wonder about whether your idea is good enough, or whether someone else has written your story before.
It's much easier to edit, restructure or redraft your story once you've done a first draft. So don't put it off until the 'right time' - just write!
You can find lots of other suggestions on how to start writing on this website.