The Idea - There are many ways that people come up with their first idea for a plot. It could come from another book or a movie. Some find it from their personal experiences, and others may just snatch it right out of the air while they're bored at work. My first idea came from a dream. All my ideas have come from dreams (yes, some are day dreams).
Brain Explosion - Write the damn thing out! Yes, my suggestion to you is to just start writing. Whether that be a first draft, a basic outline of the story, character synopsis, just the ending, or an overview of the concept. Once you've got an idea, the best choice of action will be to get everything from your mind/brain out on paper. What you will write, will be crap. Don't worry about it though. You just need to get all the nuances of the story out of your head so that you can play around with different aspects of the plot, character development, etc.
First Draft - Once you've got the basic story outline, start writing the first draft. I normally go from start to finish. Now, this won't be a very good piece of writing. Even Steven King has trouble with his first drafts. There will be plot holes, grammar and spelling mistakes, character flops, and parts of your writing that will ultimately need to be erased. Don't fret. Just get a first draft done.
Editing - This is where most of your book will evolve. Editing is where you will truly write your book. There will, most likely, be MANY rounds of editing. I'd start by doing a basic round of editing first. Then, once you've realized how crappy your first draft is, you'll need to do a chapter overview, and start moving around your plot so that it follows a more climatic flow. If your first chapter isn't catchy, then find where the action starts, and use that as your first chapter. Many readers will throw your book aside if they are not hooked in the first couple pages.
The Internet - Since you've decided by now that this book is not going to just be read by yourself, you'll need to start thinking about book marketing right away. You could start by gaining a following on sites like Wattpad or Smashwords. You could post a first chapter (only when you know that it is almost perfectly edited). I would think about getting a blog or website. Definitely sign up on Goodreads, and start interacting with other authors in your genre. There are also many Facebook groups that are helpful for first-time authors. Also, you should gain followers on Twitter, Bookbub, Facebook, and YouTube.
Final Draft - Once you've gone through your 20th round of editing (that is not an exaggeration), I would suggest finding a skilled editor to look over your book. Make sure to find a specific professional who knows what they are doing or use an editing service to work over your final draft. Yes, you'll have to shell out the dough (money). Also, make sure you get a contract signed with that individual or service company that outlines what they'll do, when it will be completed by, and how much it will cost. Trust me, this is a necessary step.
Traditional Publishing - There are the big 6 publishing houses in NYC that would be great to get signed with, but it is difficult to get their attention. A literary agent will be necessary to get your manuscript in front of one of those publishing houses. To do that, you'll need to write a very intriguing query letter (a synopsis of your story) that showcases your writing acumen, and the greatness of your book. Find a specific literary agent that represent books in your category or genre. Make sure to look at their specific submission guidelines.
Self Publishing - Now that you've received numerous rejection letters (I've got about 50 so far), you may want to think about self publishing. This is the part where you need to decide how much you want to be an author. Now, after numerous bottles of wine and pitty parties, you are now ready to be a self published author! Yes, this is the more difficult path to author success, but a very rewarding journey. Start by finding a self publishing service. There are many out there, but I've found that Createspace is the best for me. They were acquired by Amazon, so it gives you a direct platform to distribution through Amazon.
Cover - You know that saying? That one about judging books by their cover. Well.... EVERYONE DOES IT! Yes, it is true. Now that you know, your best option is to find a great cover artist to get it right. At this point, go into Goodreads or Amazon and take a look at other covers in your genre that are selling really well. Go to the best seller list and get a feel for what looks good. Some prefer a person on the cover, and some don't. You'll need to decide how best to sell your story from the cover. It's not an exact science, and you can always get a new edition done with a different cover, so don't fret. Just understand though, the cover design is one of the most important aspects to sell your book.
Formatting - Make your book interior look like a book. What I mean is that you should pay for a service to get the font, chapter titles, interior design, etc. done properly. Createspace has a service that you can utilize (its kind of expensive).
Launch - Book launches are tricky. I always suggest trying to get a few reviews online before the actual launch. You could start by doing a Goodreads giveaway or send a copy of your book to friends and family. Reviews are going to be important so that you people can see the legitimacy of your work. If you enroll in the Kindle Direct Publishing (KDP) platform, you can do free giveaways of your ebook to kindle users. Amazon giveaways are a great way to get it into reader's hands. If its a part of a series, and you've got the other books published, you can price the first book as perma-free, which will draw in more readers to your series.
Aftermath - At this point, you're a published author. Pat yourself on the back. Congratulate yourself. Throw a party! But now, your journey has just begun. Get ready for the gut-wrenching feeling when you get your first one-star review, or when you look at your sales for the month and you've sold one copy. Having pretty much drained your savings, you are now a broke author with only a dream. Hope will be your guiding light at this point. If you believe in the power of your story, other people will follow. There will be times that you'll look in the mirror and wonder what the hell you're doing, but don't let doubt creep in. You can never fail if you never give up.