People ask me this all the time. I spent most of my life wondering the same thing. I didn't dare ask the few novelists I met. I felt like Dorothy approaching the great Wizard of Oz. I never had the nerve to ask, even though it was the thing I most wanted to know.
I have four books in print. I am currently at work on my seventh novel. Word has spread, so I get approached all the time by people who want to know how to start their own books.
Here is my quick answer. It should at least get you started. There is no magic, so use this as a jumping-off point.
Buy a bunch of unlined index cards. You will write one scene on each. A 400-page novel of mine usually requires anywhere from 65 to 80 scenes. You'll fill them in as you go. Find a place on the floor where you can lay them out end to end. You won't get much of the scenes at first, but you can start with the three critical scenes: beginning, climax and end.
Let's use my debut thriller, A Dark Love, as an example. Do it with your idea (I know you have an idea!) as we go.
A Dark Love is a cat-and-mouse story of a young wife on the run from her abusive husband. So, the opening scene needs to take place on the day she walks out. That's one critical scene done. Put that on the floor at the top of what I call my Conga Line of cards. I knew from the outset they would have a confrontation (what's the point of having someone get stalked if they don't face their stalker?). That is the denouement, a very critical scene. I jotted that one down and put it in the middle of the Conga Line. There needed to be an ending where the heroine of the story has resolved her issue and faces a better future. I call that the Wrapup Scene or HEA (Happily Ever After). That's the final card, and you can put that at the end of the Conga Line.
So now you've got three cards with a ton of empty floor space in between. Don't panic. Just start filling them in. Your hero/heroine will undoubtedly have helpers and hindrances along the way. Trials and tribulations. Adventures. So will your villain. Start jotting these down. Each time a new character is introduced, it's another scene. And another card to put on the floor. Many of the cards between the first and the critical denouement will be scenes to introduce those characters and perhaps secondary plots.
Most of the scenes in the second third of your rough Conga Line will be action-oriented, where your characters interact, play out the drama and work toward that final resolution. Just write them down as they come to you.
Only the big major key scenes will come to you at first.
You'll feel great by the time you have a dozen cards on the floor.
By that time, you're ready to begin writing your first draft. I know, I know! It's only twelve cards and I said you'd need to have up to 80. More ideas will come to you as you write. This is so important, I'll say it again. The ideas will come to you as you write. Your characters will start to dictate what scenes you need to write. This is the magic of being an author, the special grace that is given only to novelists. It is a beautiful and mysterious thing, born of process not thought.
A wonderful friend gave me a crucifix many years ago with the word 'Grace' painted on it. I keep it in the kitchen so everyone who comes into my house can see it. Every time I look at it I remember that the scenes won't come to me unless I start writing.
By the time you're into the second or third chapter of your first draft (and you do have those first two cards on the floor already), you will start to fill in more and more of the cards.
The really fun part is when you start picking them up off the floor. I pick up each one as I finish the first draft of that scene. Over the course of the six months or so it takes me to write an approximate 500-page first draft, those cards become my security blanket. It feels really good to pick them up (and hopefully vacuum the floor at last).
I hope this very brief piece is of some help on the vast topic of starting a novel. It's as much practical advice as I ever received! Which is to say, there is no magic. Just jump in and get started. Good luck, Margaret