A fetus. A mustard see. A brick. A step. A word.
At first, everything has a modest beginning until ultimately growing into something complex and unique. The same can be said with a story, or more specifically a novel. If a new writer tries to write a book only thinking of it in its entirety, then the task suddenly becomes overwhelming. But take it one sentence at a time, and it starts from a more manageable bit-sized form.
The majority of people wanting to write a book often remark, but I just don't have any good ideas. I think that's because they're thinking of the whole novel - the entire, complex being that is the book. Anyone can write a novel if they make the time, but more importantly, if they narrow down an idea into one sentence.
I have come to love using a thesis sentence when devising my stories. The rule of thumb is to keep it down between 15-20 words. The thesis sentence should encompass the main theme constant throughout the story. A good example to show is a blurb from the movie MEMENTO. This is a great example of a thesis sentence: A man, suffering from short-term memory loss, uses notes and tattoos to hunt for the man he thinks killed his wife.
Yes, the above sentence was written after the movie, but it's a great example of how a story can be formed. I'm sure the writer of the movie had an idea, started simple, and then expanded. The same can be done for anyone. Think of something, anything, something that you might find intriguing or fun to write about, then write a short sentence summing it all up. After that, expand the sentence to a paragraph. Then expand that paragraph to a page. Then that page to a chapter. Then that chapter to a novel.
See, it's that easy. Lol! But seriously, if you are serious about writing a novel, don't worry about the finished product until you've narrowed down the basic premise of your story. Things will start to flow from there.