How do I format a book?

I am editing my book, which is a contemporary romance story. It is still far from being finished. However, I would like to start formatting it to the final 'shape' somewhat soon, as to make it closer to the end result. What is the ideal format? What font and size should it be? And how much space between lines? All those types of details. Does it depend on the author or the type of book? Or are…