Should you write an outline for your book? This question is of interest to many early writers, and for good reason: building an outline and proceeding without one imply very different styles of writing.
Planning Your Trip
Having an outline is attractive the same way turn-by-turn directions are: it tells us where we are going. A clear roadmap lists the steps to follow so we can keep from getting stuck and wondering what comes next. For some of us, not knowing what the next scene or chapter will entail can be paralyzing. We like our flow, and we want the creative juices to run uninterrupted.
Going on an Adventure
Jumping with both feet into the project while dispensing with extensive planning is also an attractive option. We can let the characters and their narrative take us where they may, and enjoy the ride. We may have an idea where it will lead, or we may know exactly where we are going, but we intend to discover the land in-between.
Structure and Rigidity
What an outline gives us is structure, but that structure is not rigid. Perhaps you want it to be, and if that process works for you, then by all means use it. However, preparing an outline does not mean we are required to follow it. An outline is an evolving framework, and it should evolve. It is next to impossible to think of every detail before writing and to have everything make perfect sense in the end. As such, even for the most meticulous writers have to make adjustments, I suspect.
Likewise, if we discovery-write (thank you Writing Excuses) an entire draft, its structure will likely require quite a bit of work. Revision and the next draft will do wonders for the coherence of a narrative work and its effect on the readers.
In the end, the answer comes down to our individual preferences. That being said, if you are asking yourself this question, and just want to know where to start, here it is: start with a loose outline. It may lead to the ending of the story, or it may not, but it should include important events or sections in semi-logical order up to a certain point in the story. If there are multiple points of view, try deciding which character(s) will relay which parts.
Once that is done, start writing. You will find whether or not you like this process, and, crucially, if you need a more detailed framework or cannot stand even this much planning. Finding what works for you is the most important part of writing. In order to know what that is, there is no substitute for experience.
With that in mind, get scribbling.