1. Read a lot, in many categories. Read the award-winning and bestselling books (which are not always the same books) in your genre of choice, and also the ones in other genres, and the books your favorite author recommends, and non-fiction, and humor columns, and Savage Love. Read lots of short stories, too, especially if you want to write short stories. "Year's Best" anthologies and single-author collections each have their merits. Read your genre's magazines--this will teach you lots of things, especially what kinds of stories each one publishes. Remember to read the page that tells you the names of the editors and contributors, so that you begin to get a clue about who does what in the industry.
2. Also read lots of books about writing, especially the ones by and recommended by authors you like. This is a separate step because it's very important.
3. But don't spend too much of each day reading. Don't forget to actually write. More about that later.
4. Also remember to get out of your house and have a life. No life = nothing to write about.
5. Learn how to generate and develop ideas that contain conflict, options for the characters, and multiple possibilities for resolution. I recommend two books for this learning process, and they are: From Idea to Story in 90 Seconds by Ken Rand, and What If? by Anne Bernays and Pamela Painter.
6. Don't ask published writers where they get their ideas. They don't like that. Most of them will either tell you that ideas are everywhere, or that they get them from an idea service in Schenectady.
7. Go to conventions where writers will be, though, and attend panels about writing until you've heard all of the topics at least twice and you know exactly what all of the authors will say. You'll eventually get to be on a panel, when you're published, so please don't be that person in the audience who rambles for five minutes at a time about their own pet writing project. You're there to listen and learn.
8. Definitely go to the parties at the convention. I will soon post a whole separate thing about the social skills you'll need for this. The party process has nothing *and* everything to do with writing, and the sooner you embrace that, the better off you will be in a cosmic, all-encompassing sense beyond my power to explain.
9. Speaking of things you should embrace sooner rather than later, there is the matter of writing fiction every day. You really have to, within the boundaries of human imperfection--there will be days off, but try to keep them to a reasonable minimum. Writing other things can contribute to your ability to write good sentences, but writing some fiction every day is better, and it keeps your head in the game. I like having a minimum word count to write daily; 1,000 words a day has already made changes in what I understand about what I'm doing, and it also leads to about a short story a week (rough*) which I can then edit into shape, and that's 50 stories a year, baby! It's best to get that done as early in the day as possible, because you can then either bask in your accomplishment and play Sims 2 guilt-free, or you can write more. Either way is a win, as far as I'm concerned. It really helps if you have the idea-generation thing under control (see step 5), and a baby-name book for quick character-naming.
* A rough draft can be the most crappy writing ever perpetrated by a human being, and in fact, it helps me to get started on my daily writing if I give myself permission to flail at the keyboard like a drunken monkey, if necessary. Some people will argue about whether it's better to make the first draft as clean as possible to save time, and I argue that it is not, for me. That slows things down, takes out the vitality and spontaneity of my writing, and doesn't leave me with as much to work with later, when I have more objectivity about the story. Editing is a great and glorious process. Embrace revision! (Some of us are lucky enough to experience multiple revisions... ;)
10. Don't ever let me hear the words "real life got in the way" coming out of your mouth as your excuse not to write. WRITING IS REAL LIFE. And if you're serious about being a fiction writer, you won't diminish it by implying that writing is not part of real life. Sometimes you can't write, and that's okay. It's just not a matter of other things being more real than writing.
11. Um, let's see... What have I forgotten so far? Oh, yeah. Have a plan. What kind of stories and/or novels do you want to write? How prolific do you want to be? Are you more interested in prestige, or more interested in money (and of course you understand that you can be *interested* in money without being guaranteed to *get* money (or prestige, for that matter))? What magazines would you love to have your work in? Are you planning to always have a day job with benefits, or are you aiming for the riskier life of the full-time writer? How much progress do you hope to make in the next five years? That sort of thing. Important. Write it down, and figure out the steps it will take to approximate it. You won't be able to control whether or not it happens exactly that way. Accept that.
12. Set up your life so that you're always seeing reminders about writing, and finding new things to learn about in your craft. Read the Livejournals of professional writers. Peruse my friendslist to find bunches of 'em. ETA: A commenter wisely pointed out that this is to be done after your writing for the day, not before.
13. Have a work ethic. Persistence, determination, and the willingness to accept rejections and keep on submitting are your three best friends. You won't know if you're any good at the beginning. Be a practicer. Recognize that, just as one cannot become a world-class violin player at the first lesson--regardless of how much one enjoys and appreciates violin music--so one cannot become a professional-level writer without years of work and passion for it.
14. Writing the middle of a novel is always awful, and you have to push through that and learn from it. I haven't done this yet. I intend to, when I get to the part of my plan where I start to write a novel.
15. ETA: Commenters have mentioned critiquing other people's work, and having them critique yours. Yes and yes. Find and join a writing group, take writing classes, and attend workshops if you can. Be sure to look into lots of options for those and find the best ones, places where you will get useful advice on your chosen genre and you will be neither shredded by nasty insecure people nor hand-held by people who are dishonest to protect your feelings. Try to find secure people who are committed writers, grouped roughly around your ability level--some better writers than you, some worse (some self-assessment required, results may vary). Make adjustments if you find yourself in a group that isn't helpful, but not until you've given the group an honest chance to be helpful. Sometimes you need kinds of help that you didn't know you needed.
Okay? So that's what I know. It's what I've gathered from dozens of other writers, and I think I'm getting somewhere with it!
What do you think of this list? What have I left out? What are your tips?