I realize most agents and publishers ask you information about your writer's platform. I looks like having a writer's platform has become an essential part of your introduction to agents or publishers. However, how do you address this issue if you are new in the field and you haven't actually published a book yet? How can you create a platform without a published book? Thank you for your feedback.
Yes, you can.
For example, the time I published my first book I already had a number of people willing to buy my book and who weren't related to me or weren't close friends of mine. It was a non-fiction book for a small niche and I was already known in that niche. I participated in forum discussions, used a common IRC channel, I blogged and commented on other blogs from this niche.
The first book was, too, self-published but it further helped to build my platform and was thus an important step towards the next book. This time, I published with a publishing house and already having 'a name' was important to be taken seriously by the publisher. (We proposed a book that was kind of crazy for this type of publishing house and we needed every bit of credibility we could muster.) And again I used my connections to find for example reviewers who had their own 'platform' and thus were multiplicators.
What kind of platform you need or which methods you will use to build it, depends on what you are writing.
It is just important to get in touch with your future readers. It isn't fair, but books sell more via having a name than via pure quality. There are perfect books out there, but nobody knows about their existence.
Connect with other writers. Take part in writing events. Talk about writing. Get known as a writer even though you haven't published yet. (Fake it, till you make it.) Take part in writing competitions. Get your name out there. Write a blog about your journey. Organise a local writing group. Organise a creative writing afternoon course for schoolkids. Twitter. Read your flash fiction at a coffee house that is willing to have you there (best organised together with a full writing group.) Have a Facebook page. Do anything that might help to get your name out there (but it shouldn't take all the time you need to write).
This Writer's Digest article has some more suggestions: http://www.writersdigest.com/writing-articles/by-writing-goal/build-a-platform-start-blogging/building-a-writers-platform