I had this question from a reader a couple of weeks ago, and more than once in the past, and it's high time I answered it.
Platform is a tricky thing, since it's largely nebulous and only defined (it seems) by what you don't have. The good news: you can actually build a platform, and in many cases cobble one together out of what you have.
This is going to be a long post, so here's my plan for what I'm going to talk about (so you can skip to what's relevant to your case):
1) What is platform?
2) What kind of platform you should focus on if you're writing a debut novel?
1) What is platform?
To define platform, I'm going to quote heavily from an earlier post:
The ideal author platform consists of all the below points, but in the real world we rarely work with ideals, and in most cases we can make do with a combination of some platform elements.
Passion and knowledge about a subject. Since it's nonfiction we're taliking, you have to be willing to do good research and to write evocatively/provocatively about your topic. If you're a really great writer, you have the good book author trifecta (knowledge, passion, talent!). All the best books have the trifecta but some of the best books don't actually have other platform (although that's harder, I'll admit).
Academic qualifications. This is crucial for any prescriptive books--that is, if you're giving health, self-help, or relationship advice, you really should have a doctorate in the applicable subject--an MD is absolutely essential for most health books; a psychology degree is better for self-help. A social work degree or spiritual counselor training (for example, a priesthood or rabbinical degree) helps a lot for self-help and relationship books. Josephine made a good point about this--a couple extra classes in a topic go miles toward establishing you as an expert. Not everyone has the time and money, but if you have academic qualifications already, consider capitalizing. (What did you major in in college? And who cares that you haven't dabbled in that topic since? Could you pick it up again now, with maybe a little work?)
Writing creds. If you've published before, you're more likely to get published again (in fiction and nonfiction). So dabble in things like articles. Everyone needs informative copy in their publications; master a small topic, write an article about it, become an expert that people start consulting, and next thing you know you've been published in NEWSWEEK and that looks great in your author bio. From this standpoint especially nonfiction is the route to fiction. Publishing an article on homeopathy in LADIES HOME JOURNAL actually looks better to people on my end than publishing a short story in a literary magazine--even though you're submitting a novel. Think of Mark Kurlansky and his topical history projects. He was an article writer for 30 years before making a killing with COD and SALT.
Connections. Do you know Oprah? Are you best friends with the NYT Book Review editor? Did Natalie Portman or Jerome Groopman or J.M. Coetzee promise to endorse your book because you used to date each other's sister/brother? TELL US. Because those kinds of things become platform. Hence Seinfeld's wife. Whose real name no one (myself included) seems to know.
2) What kind of platform you should focus on if you're writing a debut novel
There's a rumor going around that you don't need any platform at all to publish a debut novel. This isn't exactly true, for the simple reason that (per the definition above) if you're working on a novel you probably don't have zero platform. Furthermore, if you're a writer, you're probably also a spin artist, and you should be able to take what you have and make it work for you.
Just a note, though: the bigger your platform is the better you're going to do, regardless of the quality of your book. So while you're working and waiting, as we all do, make your job easier for yourself, and then your agent, and then your editor, by doing what you can now to continue to grow your platform.
What's going to help you most as a would-be novelist is our writing creds. Start putting together a publication list now--where have you published? Think back to college, even high school. List everything.
What about online? Do you have a blog? If not, think about starting one (I tell all my authors to blog a little--not so much that they can't find time to do their own work, but enough to establish a little net authenticity and make sure google searches redirect to their sites). Because also then your blog might start getting excerpted or cited on bigger sites, and then those become publication credits. One of my authors started a blog at my encouragement and is now frequently quoted by major national (print!) publications as an expert on her subject. Platform begot platform right there (as these things are wont to happen).
Submit. Write some articles for your local newspaper. Seriously. Can you? I bet you can. Because then the next step is your state newspaper. This even you, Stay-at-Home Mom, can do. There are also newsletters that will cater to an audience of people like you--one of my stay-at-home mom friends got a book deal after she became a regular columnist in an online magazine for...stay-at-home moms! Seriously, if you try writing and submitting some nonfiction stuff, you'll find more open doors than you will for fiction.
I hope this helped. Let me know if I can clarify anything.