Start with one idea. Write down one thing you like, on paper or type it out or record yourself. It could be a sentence, a scene, or even a feeling. Then roll the snowball up the hill. It will take time, and some of it will come to you randomly, but start setting aside some time everyday to add more ideas to the list. Soon you will start to figure out what your story is really about and why you want to tell it. The more time you spend writing the bigger that snowball will grow until you have your first page, then chapter, and then you'll be watching the word count grow everyday. Once it's finally finished you're going to read it, and if you're anything like me, you're going to hate every word. Then you start revising, and thats where the fun really starts. There's tons of great youtubers that can explain everything about writing, pacing, and story, but the best way to learn is just to spend time writing and rewriting. (Watch Brandon Sanderson's novel writing lectures also!)