My advice is to start writing. Write small stories first, and learn how to put together a cohesive story. To have a good story, one needs to decide on how the story is to be structured... is it just a series of random events? Does the protagonist have an arc, where he or she begins with a flaw and learns how to overcome it? Does he or she grow in some way? become a better person?

Have a solid ending in mind when you start. Then, built towards that ending as you go. Try to avoid using "clever side stories" along the way that don't progress the story towards the ending.

Me, I like building an outline, as you would when writing a term paper or something. One of my favorite authors describes taking writing classes where he thought the professor was an idiot and the advice was not good advice. He wrote several things and submitted them to publishers, only to get shot down each time. To prove the professor wrong, he followed the advice exactly, and when that was submitted, it became his first published work.

He said that he uses a sentence that is essentially ,"the hero's goal is X, but cannot fulfill that goal because Y is standing in his way. He must do Z to defeat Y so that he can achieve X." Fill in the X Y and Z and you have your story.

When I write anything, I have the basic term paper format of Introduction, 3 sections of Main Body and Conclusion. That's five basic blocks of writing. In the 3 sections of Main Body, I break down each section the same way, with an introduction to the section, 3 areas of investigation or challenge, where questions or challenges are posed all relating to the same theme, and a conclusion to the section. Within each area of investigation, I have the challenge and research or data (or in fiction, actions taken) to resolve, refute or support the question or challenge.

This gives me a main story arc, with 3 main phases, and nine supporting challenges to overcome, with an introduction and conclusion at either end. Flesh out the first part with a good justification of why the hero needs to go on the story arc, with some character introduction and world building, and the last section is the "prep and execution of the resolution of the book." If you add in the idea of a Hero's Journey story arc and build around it, you can fill in the details as you go, and the story kind of writes itself.