Everyone has their own motivations for writing a book. You may just have an idea in your head that wants to get out. You may just want to tell a story. Or you may have a big, giant message that you want people to get. Or it could be a little bit of both. But how exactly do you figure out what you're trying to say, and once you do, how do you get that point across?
The whole "bigger picture" thing may not come to you right away. You may even get through your entire first draft without having one. You may never have one, and that's ok, too. I've always thought that my main motivation for writing a book is just to entertain. I want to write a book that at least some people will like (and hopefully most people won't throw rotten fruit at me when I walk down the street). Not every story has to be complicated, or have some sort of message to get across. I call a lot of my story ideas "fluff" stories, maybe just because they feel a heck of a lot easier and lighter than my main WIP. Fluff can be great (and lots of fun to read!). But not every story is fluff. Some stories are a bit more complicated.
That isn't to say that I think my book has one big message. If people assumed that, they would probably get it wrong. I do think that it does have several smaller messages, some of which I'm still trying to figure out. So how do you figure it out? How do you know if your story even needs one? And then how do you get those messages across?
It's probably something that you'll figure out along the way, or maybe you already know what your message is before you write the first word. Every story is different. It all depends on how subtle or obvious you need your message to be. How important is it that the reader gets it? If they don't get it, will the story still work? All of these factors will go in to your thought process as you're figuring it out.
And there are plenty of ways to get a point across. Think about all of those English papers you had to write in high school. Themes, motifs, symbolism, characterization. When you start trying to crack a story open, you can figure out what the writer was trying to say. So when you're the writer, utilize these tools. Put some symbolism into your book. Have a character represent something bigger. How often you do this will depend on the story, and you obviously don't want to overdo it so that you're hitting the reader in the face (didn't I talk about this before...?).
The first step in telling a story is to just tell the story. The next step may be to figure out what it is you're trying to say. The first person who has to "get it" is you.
Ever write a story with a message in it? How do you utilize themes and symbolism?