In nonfiction, credibility is your platform. If you've done extensive research into your subject matter, are a known expert in that field, or have a PhD after your name, you probably have credibility, and for those interested in your subject, you have a built-in audience, or instant platform as well. Consider the well-known radio advice personality, Dr. Laura, for instance. In addition to her PhD in physiology, she has done research in human behavior, religion, and has expressed her views on her radio program. Because of this, she has automatic credibility and platform. Anything she wants to write would be immediately picked up by a major publisher.
For fiction, credibility and platform are very different. A writer does not need to establish himself as an expert in some field of endeavor, though it can help, but more needs to write material that is at once credible and entertaining. To be credible, the writer needs to research the subject matter, the setting, characters, etc., and must be able to craft a compelling story. That, to me, constitutes credibility for fiction. Platform, then comes to the writer as his writings gain respect and following from his readers.
At least that was how it used to be.
Today, writers, in addition to writing and crafting compelling stories, are also expected to be marketers, or salesmen if you will. Many writers, who are timid by nature, find this expectation difficult to bear. For whatever reason, publishers are unwilling to expend a lot of money on publicizing works of new and unknown authors who lack platform. Thus, it is important for a writer to become also a salesman. One way to do this is by establishing himself in the writing community as a known commodity even before his writing is accepted for publication. If he can also establish himself with his genre's reading group, i.e. young adults, readers of fantasy or science fiction, mystery readers, etc., he will be that much further ahead when his work is finally accepted for publication. So, where do you find these reading groups? I'm not an inexhaustible source of information on that subject, but think about places you've been, organizations you've been part of. Consider old school friends, people you may not have heard from in many years. I'm finding more old friends nearly every day on Facebook, ClassMates, Twitter, etc. I'm also a member of several writers forum web pages. In all my signature blocks, I've got a reference to this blog, and as soon as my book is accepted for publication, I intend to contact everyone I know, and hopefully word of mouth will take care of the rest. And, if I can get a small allowance for advertising, that will be a help too.
Happy writing and reading.