For me, an idea for a novel always begins with a character. Some of my characters are carefully planned, others just seem to show up out of nowhere. When I do writer-for-hire work, characters created by someone else are assigned to me. However they get here, the characters are the catalyst for any story I write.
There are any number of character outline sheets writers use*, but every character I imagine takes form by answering three basic questions: Who are you? What do you want? What's the worst thing that I can do to you?
StarDoc: I'm a doctor, I want to do no harm, start a war over me.
Heat of the Moment: I'm a cop, I want justice, make the man I love a criminal.
Blade Dancer: I'm an orphan, I want a family, make me a pariah.
When I have those three answers, I've got my protagonist, and the foundation for his or her novel. This is also known as the novel premise.
Now I start to get the details that become the framework for the novel, i.e. my character became a doctor or a cop or an orphan because (fill in the blanks.) What goes in the blanks becomes my character outline and my backstory.
Finding out why the character wants to do no harm or justice or a family is strongly related to backstory, even if it represents a complete departure for the character, because what drives us now is always related to the past. Knowing what the character wants also brings me to the present, and what the characters is doing to pursue that desire, aka the central plot and setting.
The worst thing that can happen is simply the keystone of the novel's main conflict.
I can almost hear grumbling out there, but hang on. Your questions may be different from mine, depending on the writer you are and what interests you about your characters. Thus Who are you? might become Who were you? or How were you damaged? or What do you symbolize?
Your plot and setting may not revolve around what your characters wants, but are driven by other sources. Not a problem. Whatever the case, you still have to define them.
Same goes for your conflict: it doesn't have to be the worst thing that can happen to your character. It can be the best thing. It can happen to someone else. Whatever it is, however, it should affect your character in some fashion.
To give you a more visual sense of how I plan, I was once taught a meditation technique for when I was upset, in which I visualized myself as a lotus flower (no snickering, I'm serious.) I was the center of the flower, and everything I cared about or was upset over were the petals; least important on the outside, vital stuff at the heart of the flower. In my head, I took my flower-self apart, examining each petal, seeing the beauty and the flaws, and accepting what I saw. By the time I got to the center, I was calm (this really works well for me, btw, if anyone wants to give it a whirl.)
When I plan a novel, it's a little like doing the lotus meditation in reverse. The character is the center. The answers to those three questions are the first petals around the center. Everything else grows from them and becomes the flower, which is the novel.
*I do use character outline sheets to work out facts and backstory. One that I wrote for Jory Rask, the protagonist of Blade Dancer, starts on page 27 of my revised Novel Notebook (.pdf format; warning, contains major spoilers about the novel.)