The truth is that the actual writing of a book is only a small part of being published. Anyone can write a book. But not just anyone can write a book well and not every story that wants to be told is worth telling...at least not in the money-making sense. So how do you know if your book is one that is written well, one that others want to read? What DO you do now?
It's as simple and as painstaking as that.
Learn the craft: take classes offered by a local community college or adult ed program; enroll in an online writing course (check out the Institute for Children's Literature); read books on writing (Anne Lamont's Bird by Bird is a great place to start).
Learn your chosen genre: Find out what has already been published in your chosen genre. Is there room for more or is the market flooded? The library is your friend. So is Goodreads. And Amazon. Visit bookstores. Find out what is popular and what's been written to death.
Learn from other authors. Read books in your chosen genre. Write what you love to read. Read what you love to write. Pay attention to voice, character development, plot, pacing, style.
Learn from local writers. Join a critique group. Be vulnerable. You'll feel like you've put your soul into the hands of strangers. That's okay. Your fellow writers are there to help you. Listen to their feedback on your work and on the work of others. Don't take their comments personally. Doing so could destroy your chances of publication - or worse, make you stop writing.
Learn the industry: Join SCBWI (if you're writing for children through young adult). Join your local or regional writers' guild. Go to the meetings. Go to the conferences. Meet other authors. Meet editors, publishers, and agents.
As you learn, you'll figure out what your next step is. You'll have the information you need to make the decisions that are best for you.
Being published is not the end-all to writing. There is much joy to be had in the journey. At one point, I'd forgotten that, so I took year off from all outside writers' groups, conferences, and newsletters. I wanted to focus. I wanted to remember why I started doing this in the first place. I wanted to recapture the joy of the journey.
If you're just starting out - or if you've been writing for years - treasure the path before you and every step, every lesson, every new friend you make along the way. The journey won't be over until you decide it's over, so take each moment and cherish it for the gift that it is.