So you've got a killer idea and you think you want to try your hand at writing but you don't know where to start. Do you hand write it? Do you use a word processor? What processor to use? How do you format it?
First, and most importantly, JUST WRITE. That's the most important thing. Write as often as you can, and write as much as you can. In order to craft a story, whether that be a short story, a novella, or a novel, you have to put the words on the page. That can only be done when you make a conscious decision to stop procrastinating, stop worrying, and just get those words down. The first draft is never good. The wording of the sentence can always be better. There are always better word choices and sentence structures, but you can't polish that turd until you get it out.
As you become a better writer, those first drafts will be cleaner, but they still won't be "good". You will always improve your craft, but that too can only be accomplished by JUST WRITING.
There are a few ways you can tackle the actual physical task of getting the words down. Obviously, pen and paper is one way. I've tried numerous times and ended up abandoning this method, not because I don't like it, but because my hand writing looks like a kindergartener high on hallucinogenics wrote it. If you think it's something you might be interested in, there are studies that show your brain is activated in ways that simply don't happen when typing, so the method itself is very conducive to the craft.
But what if you want to type it out? Even if you hand write the first draft you've got to type it eventually. If you're on Mac, pages is an excellent option. It's easy to use and chances are if you're on a MacBook, you have an iPhone and the pages IOS app work's seamlessly using iCloud to allow you to work on your phone if you want to write like that to get some words in. Personally, after trying other processors and methods, I've settled on pages.
Mac users also may consider Scrivener, which is a word processor built specifically for writers. Many people swear by it, and it has some great features, but with great features comes a learning curve. For me, Scrivener felt like it complicated the process too much.
When in doubt, 'Ole faithful, Microsoft word is always a good option, as well as Google Docs. I wrote my novel, The Warrior Retreat using Google Docs.
For those of you who simply can't manage to write without getting distracted, portable processors that don't require an internet connection are a thing, and people seem to really enjoy this method. You don't have to worry about checking your twitter 856 times per session when you don't have access to it. Freewrite is the one I've heard of before. I can't speak to how well it works, or ease of use, but I believe this is the one that is considered the "brand".
In the end, the method and processor you use should be the one that allows you to be the most productive, because what matters most is writing as much as you can, as often as you can. Both of my books I used the mindset any time any place even if it's only a few hundred words. You can see how The Warrior Retreat turned out, and Woe To Those Who Dwell On Earth was also written in small slices of time over the course of a year.