What everybody else said... Also, being a fast typist helps... Be sure to write down/record ALL of your good ideas - I've had a few script/book/story plots pop into my head, had fun tweaking them, then was CERTAIN I'd remember them when I got around to writing them down, and POOOF! Gone... Review your ideas; be kind, sometimes a bad-sounding idea works out well... I usually rough-out a plot outline - btw, I refer to my writings as 'plots' because that's usually the starting point for a book, script, screenplay or story... Though sometimes a book/story starts out with an opening sentence, a scene occurring midway thru the book, and so on...
Oh, and here's the acid test. Keep developing your idea/story. Do it REGULARLY - and this means MAKING time every day, every two days, or twice a week, to work steadily on your writing - and I mean picking out one, two, or maybe three ideas that are forming most rapidly or most thoroughly in your mind, and GET THEM FINISHED.
This is my big problem. I come up with so many ideas that I often can't focus on ONE project, get it done, and move on to the next one... Focus on ONE or two projects, get them DONE - no second-guessing, btw, and have a friend read it - preferably one in the general demographics of your 'target' audience, i.e., sci-fi fan for science fiction, murder-mystery fan for mystery writers, and so on... Tell them to be brutally honest about the book/story, but remember that they aren't buying your writing, and a publisher might like it more/not like it at all, which brings me to the most important test...
Now comes the ACID test!!! SEND IT OFF TO A REAL PUBLISHER! And then comes the even more difficult part - KEEP SENDING IT OUT while you work on the next one - DESPITE THE REJECTION SLIPS! Remember, Dr. Seuss received 23 rejection slips before he was accepted by one publisher, and since there are many more publishers - as well as authors - in the modern market, you may receive more rejection slips before you're published.
Someone [don't remember who, unfortunately...] once said, "The most important thing about achieving success is to just show up..." paraphrased; in other words, to be consistent and believe in your talent enough to consistently keep putting it out there despite rejections.
Now for the 'nuts-and-bolts' advice: Be sure to KEEP A COPY FOR YOURSELF, and DATE YOUR COPIES. Be sure to SEND YOUR SUBMISSION VIA dated, REGISTERED MAIL that requires A SIGNATURE, so you can prove you submitted it to said publisher in case of plagiarism - and yes, it does happen nowadays, too. Check out the possibility of getting a literary agent - and this would be an acid test, too - because you will probably have to 'court' the agent - show him/her that you have the talent AND THE RELIABILITY AND CONSISTENCY to keep producing. 'One-hit-wonders' aren't as profitable as hack writers that churn out lots of books; look at the success of Stephen King and the quality - lack thereof, in my opinion - of his works...
My opinions... None of my stuff published yet... Zid