Publishers don't just want you to be able to write the book. They want you to be able to sell the book. So when you write your book proposal to publishers and agents, they will want to know your marketing skills and willingness to get out there and sell. Unfortunately, most authors are introverts, so this presents a problem. Luckily, a lot of marketing today is online through social media and blogs (i.e. blog tours), and that is one solution for authors who have trouble speaking in front of large groups.
Here is a great article about 3 Questions you should answer when writing a book proposal, which should help. And don't worry if you're not saavy about marketing yet. There are lots of articles online about how to market your book. It's pretty easy once you get started. One last word of advice...join an author group. They will help you with the ins and outs of book marketing and also be willing to promote you on their blogs, once your book is published. It's a great first step, and one you can tell your publishers about when you propose your book to them. Good luck!