I think just diving right in and writing something is the most important part, you can always go back and edit it once you're more practiced, if it turns out to be something you like. I find that when starting something new, plotting it out on the three-act screenplay structure helps me get the main bones of it together. Ironically, I don't actually use the screenplay structure all that much for screenplays, I tend to just dive right in. But for prose, it's really useful to be able to lay down your plot visually then go in and chop it up into chapters so you can focus on one segment at a time. That's the most simplistic version and the one I always use.