Bear in mind also that the market research is going to be up to you and the “platform” is going to be up to you. I’m assuming that your book is nonfiction.
One initial step might be to go to the Internet and also to an actual bookstore and see what, if anything, there already is out there similar to what you have in mind. What will your book offer? How is it different? What will the selling point be? Who’s your audience? Who will buy it? This is part of your marketing.
Another is to start a blog. Really. A blog on a topic related to your subject matter, or even a blog that could become your subject matter—reedited into a book. The audience you gather is a big part of your platform and gains you a following before you ever publish.
Self-publishing is also becoming respectable these days, but there are a few big cautions with that: (1) you do all your own selling and promotion, (2) you have to be careful about choosing a service, and (3) writers are not capable of editing their own books and deciding when they’re ready to go to press. You must have an editor’s help.