One of the problems of beginning work on a story is that it is so easy to get bogged down in details. Who is this character? What happens here? How do I get between this scene and that? What sort of atmosphere should I go for? It's very easy to feel as though the whole thing is too complicated a mess to get control of before the firt key is ever struck.
The answer to this is to remember that when you start a story you don't begin with characters or moods or scenes or any of that. You begin with the plot and that is very simple and direct.
At its most basic, a plot is just a matter of stating what it is that you want your protagonist to achieve, what stands in his way, and what the end of your story is–and not necessarily in that order. Many a classic story has been written backwards. In 1945, John Nanovic called this the "Triple-O Way", which stood for Objective, Obstacles, and Outcome. Nonovic claimed that all you need to so is figure out what the protagonist wants (Objective), why he can't get it (Obstacle), and who it all turns out (Outcome). Once you have those, everything else is mere elaboration.
I'm not sure if this is the best way to approach a story, but it certainly is one of the most effective. I hate to think of how many ideas I've had that seem really great, but I could never find the right way to express them. The reason was that there was no dramatic reason why what I had to say was worth saying, so I would spend months or even years trying to pound square ideas into round plots. The answer is that I had the process backwards. Sure, the purpose of the work was to get my message across, but I needed the framework to put the message in . That framework was the plot and until I'd settled on that, the message would never fit anywhere comfortably. This is particularly true where the story was something very structured like a farce or a murder mystery.
The key is to keep it simple, keep it focused, and and figure out what stands between hero and home.
Once those three are sorted out, the rest will follow.