Once you’ve chosen what your fiction will be about, how do you start writing? Well, this is where it comes down to the question: are you going to be a plotter, or a pantser?
Being a plotter has worked for me a lot of times, if I need to figure out what this story even is. If it’s going to be on the complicated side and/or my initial idea is still hazy, I benefit from writing out a synopsis of how things might go. (This always gets changed later, as I do the actual writing of the book, so don’t worry about having to stick to the outline too closely.)
However, if I have a good enough idea where the story will be going, and I have no crazy magical rules to figure out, nor intricate cat-and-mouse games to plot, I’ve also successfully written in “pantser” mode—that is, writing by the seat of my pants. (Apparently that’s the origin of the term “pantser.” But I also like the idea of “pantsing” the novel; i.e., just yanking its pants down and getting straight to business without any fancy planned striptease. Not that I actually think of novel writing as sex. Although…hm. You know what, metaphors can be a serious sidetrack. Moving on.)
I don’t think you need to commit strongly to one side or the other, outlining vs. discovery-writing. In my experience, every book’s creation has some elements of both. There are parts you’ll probably have to stop and work out ahead of time in order to proceed properly. But there are sure to be other parts you won’t know about until they happen, springing serendipitously from your keyboard and delighting you with how right they feel even though you just thought of them. So ultimately that’s my advice, which is sort of anti-advice in this case: don’t worry so much about “plotter vs. pantser,” a.k.a. “outlining vs. free-writing.” If you can’t figure out where to start or where it’s going, try some outlining. If you’re eager to get in there and write and just see where it goes, do that. You’ll be all right.