@CyanoticWasp GA!
What @CyanoticWasp is very true, and it is very important for your writing to be clear and correct if you want it to be published. Your question is about starting the writing process, and there are a number of ways you can attack it. What works for me is just to start at a scene that interests me the most and work backward or forward from there, playing connect-the-dots. I carry a notebook with me and write down lines of dialogue or action as they occur to me, then I find a way to fit them into my story.
You might be more organized, and make an outline of your plot points, then start writing your story at the beginning. It’s helpful if there’s an hour or two in your day that you can devote to writing- mine is when I take the train to work, but it might be your lunch break or after you get home. Whatever works for you. Good luck!