Here. You’re going to need these.
1) A Fine Set Of Big Brass Ones. Balls or ovaries, dealer’s choice. You will need these to start, because without them, you’ll likely be discouraged by naysayers, lingering self-doubts, or the idea of putting words in order for several months with no promise* of a giant pile of money at the end. There are a million reasons not to, and before you’re done you’ll find out what most of them are and maybe invent one or two new ones on your own. If you’re pitting your will against all that, it’ll help to have the stones to follow through.
So strap on your biggest pair, because you’re going to need ‘em.
2) An Idea. Ideally, an idea that you’ve worked with a bit and which has spawned a whole colony of sub-ideas, all of which can go into creating an interesting whole. But in a pinch you can substitute a brand new Flash-In-The-Pan and see how it goes. Maybe it’ll fizzle out after a chapter or two, or maybe it’ll make a fire big enough to toast your brain from the inside. Only one way to find out, right?
3) A Practical Concept Of How You Write. Do you prefer to pick at it a little every day? Or are you the weekend binger, knocking out whole chapters while watching the Sunday afternoon games? Do you want to get up in the morning and scratch out a couple of pages while your spouse sleeps and the paper boy runs from the neighbour’s murderous Rottweillers? Or do you want to stay up past your bedtime, pecking away by the blue light of the computer? Have an idea of how you work, or how you want to try to work. This will give you discipline.
Don’t worry if it’s your first time; if it doesn’t work, you can always change it. I’m not going to come to your house and kick your door in.**
4) A Sub-Atomic Transcription-ator. Or a computer. Or a notebook. Some means by which you will take the raw, fresh squeezings of your brain and turn them into a useable form. Like making booze, you’ve got to have the right container. Charred white oak or a brand-new journal, sherry casks or a clean Word document: each will give your writing a different feel and a different flavour. Pick the one that suits the story.
5) Desire. You have to want it. And you have to want it enough that you are willing to invest something of which you have a finite amount: time.
So want it. Want it right down in your mitochondria.
That’s my list. What do you need to write a novel?
*If you do have a promise of a big pile of money at the end, congratulations on the satanic book deal for which you sold your soul.
**Unless you have coffee when I’ve run out. Or pecan pie. Big pie fan.
Bare Knuckle Writer 
One change I’d make is pick the mediums of writing that work for you. All do lend different tones and approaches, so I find myself moving between them in one project.
Really? I usually stick with one medium for one project. But to each their own. Maybe I’ll give switching a try next time I’m stuck.
Steel balls are more durable. With that I mean you not only have to have the power to resist doubts from within and without, you must also have the perseverance and stamina to do so and continue writing for what could end up being years.
I also think the ability to read what you have written again and again without getting sick of the sight of it is pretty much a prerequisite, in order to iron out as many flaws as possible. You can be sure there will be flaws, whole bloody fucking flocks of them. Or herds? Schools? Swarms? Whatever. Probably all of the above.
Have you seen the cost of steel these days?
And I think the collective form of flaws is a clusterfuck.
I have to admit I do not follow the steel prices. *hangs head in mock shame*
Clusterfuck, hmm. Somehow that leads my mind to the reproductive abilities of flaws. Parthenogenesis, big time. Very difficult to combat when you are intent on flaw-genocide. Kill the bastards! Get the females first, the rest will just die off…! Nope.
Oh, and one other thing I think is extremely useful when you want to write a novel is a good memory. Sometimes it’s not easy juggling several characters and story-lines in your head, but it helps if you can remember what specific characters did and said 100 pages ago.