So you have determined that you are indeed a writer. You just cannot help but write. The next step is to decide what the the subject of your manuscript will be. After all you are determined to become an author and being an author requires that you get published and get paid. That requires a finished manuscript.
“What should I write about?”
There is an oft-repeated axiom that states that you should write about what you know. Comments of that nature are a bit simplistic since it is impossible to write about what you do not know. The real answer to that question is that you can really only write with passion about subjects about which you are passionate. Passion is the key.
What excites you? That is the subject of your first manuscript.
Some writers start with an idea, a concept, that passion and let it take them where it will. Others prefer to outline their story and then work within that framework. Sometimes a certain segment of the story demands to be written NOW. If so write it. It will not matter that it is out of sequence. That is what rewrites are for. Use whatever system works for you. Create your own system if need be and then share it with other writers.
Just write. Write even when you don’t feel like writing. Turn off that damn TV. The program you are watching, the one that has your attention, you would not ordinarily be watching anyway. Today it possesses an inordinate amount of interest for you even though you find it, basically, uninteresting. Turn off the TV.
Walk passed the refrigerator. Yes, that Ben and Jerry’s “Chunky Monkey” ice cream is in there and, yes, it would taste great right about now but NO! Walk passed the refrigerator. You do not need the extra calories anyway. Forget about the news. Forget about calling what’s-her/his-face. Forget about everything except your writing.
Drag yourself in front of that blank page/screen. You do not feel like writing right now but you must. It is called discipline and if you have none you will never be a successful writer. In fact, you will never be successful at anything difficult.
So finally you are sitting in front of that empty blankness with nothing to write. The blankness stares back at you. Stay there. Stay. Soon enough you will begin writing just because you are where you are. The words do not come easily. They seem contrived, awkward, boring.
But then you write a phrase or a sentence or, maybe, even a paragraph that you recognize as well written. Now you are writing and, before you know it, you have the beginning of your manuscript.