This is a fundamental step, at least regarding the amount of time it will take you.
Firstly, let’s define: The argument (main statement) is a series of linked events, in relation to a character who wants to accomplish something important and won’t be easy to achieve. The events must adhere to the internal logic of the story and reach a satisfactory conclusion.
To reach this, we must understand a few more concepts: the story and the plot.
The story is the development of action (where our characters get involved), in linear and chronological form. This is all that happens in your novel, telling it from the beginning till the end, in the natural order of events. However, the plot is how you decide to tell these events. It is the result of organizing the narrative of the story you have thought. And in order to build a plot you must make fundamental decisions such as: Selecting which passages will be told in the story, because in a series of events not everything is interesting and you’ll have to determine the order in which these selected passages will be told.
To this story you’ll need to link one or many subplots that will perfectly integrate the main plot. You’ll also need to create an intrigue through planting “signs” that will serve as reference points for future events, so that they don’t seem like “they have just fallen from the sky”.
Once you have completed the base argument, you will be ready to start writing your novel.