Since I hear from a lot of friends, acquaintances, and friends-of-acquaintances asking about how to get their book(s) published, I thought I'd boil down the three methods here. This post is meant as a starting point, both for making your own decision about which method to pursue, and for starting your own, more in depth research. Also note these suggestions are most useful when seeking to publish a work of fiction, though they're still mostly true when publishing nonfiction as well.
One note applies to all three methods though: If you want to publish a book, your first step is to finish writing it. It sounds obvious, but it's actually very difficult, because writing a book is extremely time-consuming and draining, and it can be hard to finish something like that without any reward along the way. So the temptation is to write a little bit, and then either use that little bit to query agents or publishers "on proposal," or to self-pub it as the first installment of something. The first way leads to a situation where (joy! happiness!) an agent or publisher expresses interest...and asks for the rest of the manuscript, which does not exist yet and you cannot possibly complete, in good quality, in the next week or month. The second way leads to readers who (joy! happiness!) got hooked on your story...but then lost all interest or got angry or swore it off when it took too long for you to post the next bit.