By this time you should already have an idea of the main story line in your head, time to write that down on paper.
Writers usually fall into two camps when it comes to plot - the Pantser, and the Plotter.
The Pantser prefers to write their stories by the seat of their pants, with little more than a character, a problem, and a path forward. The qualities of their protagonist and antagonist drag them inexorably forward toward some final showdown, with perhaps a plot twist to spice things up.
The Plotter lays out the path of the story, poring over each plot point and forcing the story into a developed path for a controlled resolution.
For either camp, I would recommend using Randy Ingermanson's Snowflake Method. This allows you to plot just as deeply as you wish, from three main plot points to a chapter-by-chapter outline or even deeper. I normally develop the plot and record it in Microsoft Excel, see Step 2a for a copy of the spreadsheet I use.
There are also several plot lines your novel can take. A simplistic one is the 'Witch's Hat' model:
The critical plot points in this story arc are a point where the Protagonist is propelled into action by some event. Thus ensues rising action as the hero overcomes obstacles until they reach a climax point, the 'final showdown' against the Antagonist. At this point, the Protagonist completes his task, wins the girl, whatever the outcome is. The Falling Action is wrap-up of additional story threads - the serial killer is hauled off to jail, the friends go out to dinner to celebrate, etc.
The resolution is the final state of the hero. I've left it on the same level as the beginning, but the hero should end up changed. This would be the end result of the Character Arc. What has changed in the Protagonist that makes him a better (or worse?) person.
Another model for plotting a Story is the Mountain Range Diagram.
This introduces the idea of the death of a vision. In this model, the hero's critical flaw, like overconfidence, pride, naivete, etc, is exposed and exploited by the Antagonist, or simply becomes the hero's downfall. The hero ends up with a death of his original vision, and must come out of the ashes redefined, with a new goal or new definition, to overcome his flaw and defeat the Antagonist.
After the final conflict, the hero should emerge with the fatal flaw resolved.
Another Plot model is the Multiple Thread.
Some stories, like Darkness Hid by Jill Williamson, use a multiple thread model. This model assumes multiple Protagonists, all with their own story lines, all with their own character arcs, fatal flaws, and strengths, separately carried through action until their threads intertwine. Alone, they cannot complete their goals or defeat their foes. However, when joined, they complement each other and can overcome the final obstacles. The Inheritance series by Christopher Paolini also uses this plot method, as does The Wheel Of Time series by Robert Jordan.
The final plot diagram I want to discuss is the Hero's Circle.
The Hero begins the story living out his daily life when a problem is introduced that will force him to leave the comforts of home in order to accomplish some goal. He must eventually, based on some death or loss, make the decision to depart on the road to adventure, and so he heads out.
Along the way he encounters something dramatic that will redefine his view of the world, and suffer the death of something precious to him. Assisted by an unexpected mentor, he will be redefined based on a revelation provided by the mentor or contemplation, and with determination and his redefinition, he will complete his new task (or the old one) and return home victorious and changed.
The Hobbit, The Lord of The Rings, Star Wars, are all examples of this model.
Non-Fiction
One who writes non-fiction might wonder why a plot would be important to their work. Big surprise, it's actually not. But outlining the work is vitally important to completing it and keeping it structured for consumption.In college English classes (And in high school too) they teach you when writing a term paper that you need to:
Write a Proposal Statement (what your main point is in the paper)
Create an Outline (Main Points)
Flesh out the Outline
Head to the Library with Index Cards to Research, identify sources, references, etc.
Write a Rough Draft - Intro, main points, Conclusion, Bibliography
Actually, sounds suspiciously like that Snowflake Model. And it is.
For example, let's say you are writing a devotional on Marriage and Family.
You want to cover topics like:
Dating and setting Boundaries
Selecting a Life Partner
Planning for Married Life
Raising Godly Children
Your topics become your sections (Like Acts in a Play)
Subtopics become Chapters: (Like scenes in the Act)
I. Dating and Setting Boundaries
---A. Why Date at All?
---B. What Constitutes a Date?
---C. What Boundaries Should we Set?
---D. Accountability in Dating?
etc.
Whether writing a 30-Day Devotional or a Textbook on Underwater Basketweaving, these steps will keep your work structured.