Many very successful authors say they never outline a story before writing it. Stephen King says he doesn't, and I certainly can't argue with his accomplishments. Dean Koontz also says he doesn't outline. More power to them both. That method doesn't work for me, however, and I don't recommend it, especially for a beginner.
There have been a handful of times over the years when I've started a book, made it as far as the first couple of chapters, and then didn't know where to go from there. I'd written myself into a corner and didn't know how to get out of it. That's because I'd tried to write without first creating an outline. It's in the outlining process where I work out any problems, and make sure I have a clear path as to where the story is going.
Remember when I said the plot of your story is its skeleton? Well, the outline is where I figure out the plot.
I start my outline at the opening scene, where I want the book to start. I have the general story idea and the theme in my head, and I find it helpful to write these down in a paragraph or two. But when I start my outline, I see it happening as it will appear in the book. I write the outline in the present tense: "Joan, a recently laid-off librarian, arrives as the run-down mansion that she has inherited from her grandfather. Entering through the front door, she sees that the place is a mess." And so on. It gives me a sense of immediacy, of being in the scene.
(Don't forget your 3-Act Structure. It'll help you keep your outline on the right track.)
Most likely, when I start the outline, I have only the germ of an idea. But there's something about writing down the action as it happens that generates more ideas, and they start flowing like crazy. Soon I reach the point where I'm writing furiously, trying to keep up with my brain as the story begins to take shape. (I always write my outline in longhand.) I'll introduce characters, inserts scenes, move the story forward in a way that often surprises even me. This is the creative process at its best.
What if, halfway through my outline, Joan's crazy Aunt Helen, who has come to stay with her, runs out the back door and shoots a bear? Surprises are good. Even shocking things are good. But you have to make sure that the shock is still logical. At that point I'll go back to an earlier scene in my outline and make sure I insert some tidbit of information that will make the shooting of the bear a valid occurrence. Maybe I'll have Joan and her growing family sitting around the table having their morning coffee and listening to the radio. Maybe there's something mentioned on the radio about a circus coming to town. At the time this didn't seem important; it was just background noise. But when Aunt Helen shoots the bear, the reader thinks, "Aha... the circus!" And don't forget the gun -- I'd better mention something about that earlier in the story, too.
These early bits of information are called Foreshadowing. But what if you want to throw your reader off the track? Can you foreshadow, but have it be false? Sure you can. It's called a Red Herring, and you'll see it used especially in mysteries. In your mystery you don't want to make it too obvious who the killer is, so you throw in some hints that casts suspicion on someone else. That way, when the killer is revealed, the reader is pleasantly surprised. Agatha Christie was a master of the Red Herring. Just read "And Then There Were None" some time.
These are the things I figure out in my outline. I weave events through the outline, creating scenes, adding subplots, bringing in new characters, building suspense. By the time I get to the end, it's admittedly kind of a mess. Notes have been inserted here and there, sections have been moved around. But, rough or not, it's a complete outline and that gives me the foundation I need on which to work. I'll then go through the outline again, moving scenes if I think they need to be moved, making sure all relevant characters are given enough time, cutting those that aren't necessary and tying up any loose ends.
For a full length book, my finished outline might run anywhere from 15 to 25 pages and take me two weeks to a month to complete. A lot of work, sure, but it's worth it. It's given me a map to follow, making the writing of the book easier and more enjoyable.
"To be a person is to have a story to tell." Isak Dinesen