My answer used to be, "I don't know. Pick up a pen and write something down?" I was facing the same question myself. But now that I've actually finished a book, I can sit back and think about what was useful for me and what wasn't.
Things you need to be able to write:
1. A quiet place. I wrote most of my novel in the library when I was studying for my writing degree at my first college, and I thought about the plot while walking around the campus. I soaked up a lot of the inspiration around me, and some of the architecture there (including above-ground tunnels) made it into the book.
2. Resources. If you're one for writing guides or just need inspiration, Bird by Bird and On Writing are the books I always recommend to new authors. Bird by Bird is especially good, because it's a heartfelt book that at the same time takes a "down in the trenches" approach to coaching new writers: the author lays everything out and tells it like it is.
The Elements of Style is also a must for any writer. It's short; go grab a copy and read it. I also found Eats, Shoots & Leaves to be a hilarious grammar resource (but other people have said it's boring, so I might just be a grammar geek).
If you're looking to write an ebook, Zoe Winters' Becoming an Indie Author is a great resource as well. For news in e-publishing, J. A. Konrath's blog is a must-read.
If you're a poet, In the Palm of Your Hand is an excellent book to learn some skills and read examples of fascinating poetry.
3. Time. I did most of my writing late at night or between classes. If I had a day off, I wrote. During nursing school I could go months at a time without writing, but during the summer I jumped right back into it and worked on the draft of a new book. Nowadays I dash off notes and entire poems on my phone while waiting for the bus.
It takes some effort to find spare time, but most people have more of it than they realize. I've seen people write dozens of tweets in a single day, then type, "I never have enough time to write!" Don't be that person.
4. Good software. I once used RoughDraft, because it was a lightweight program that did a lot, especially due to its "Notepad" function (each file has a little sidebar you can use to write notes).
I had to start using Word for school, however, so now I often type in that and outline in OneNote, which I find indispensable for organizing a new book (one tab for plot outlines, one tab for character lists, etc), so I have everything in one place. If I forget what eye color a character has, or where they're supposed to be in the next scene, I can flip back and find out quickly.
5. Writing buddies. If no one you know is an aspiring writer, try finding a writing forum online; AbsoluteWrite is a really comprehensive one.
One thing that really sped up my writing was finding "betas"--essentially, people to read, and give thoughts on, my writing. My first betas were friends and siblings, and I would deliver paper copies of my chapters to them. Eventually this led to me e-mailing them, and finally using Google Docs to create files that everyone could read together and pile notes on. It's a lot of fun, and it's a good motivator to keep writing when you have instant feedback.
6. An idea. Whether you're writing poetry, a short story, a novel or anything else, you really need to find something you believe in to write about. This is the most important and yet elusive part of writing. My ideas usually come in the form of a thought that grabs me and won't let me go.
The original thoughts for my book Flyday went something like this: What if a girl had a time machine that could take her anywhere she wanted to go? And what if she encountered a man accused of murder, and found that her future actions might have caused it? I wondered where the story went, and I wrote it.
Someone once told me, "I write because the books I'd want to read aren't written yet." This is a good way to think about writing. What would you want to read if you saw it on the shelf? Write it.