We might sort the puzzle pieces next into colors that go together, or begin the structure by finding the edge pieces. Start sorting out the things you know you want to happen in your story by putting them in the order that makes the most sense for now. A few will be shuffled around later on. For example, if your main character's goal is to have his horse win the championship race, you know that training comes in early. Put as many of the things that you wrote down into an order. This is a loose outline--whether or not you consider yourself an outliner. This planning step will save time later.
Now you want to put a few pieces together on paper. Go ahead and write those scenes that you've been dying to start on, the ones you already visualize strongly. It's like putting one section of the puzzle together. It doesn't matter yet how this section will connect to another. It's stimulating to see something emerge, to show progress. Yes, pieces of the section will still be missing, but you're writing! Remember, this is a rough draft.
For those who like more structure to their outline before writing, there are all kinds of helps beyond the scope of this post. Time spent gaining knowledge will save time in the long run. Other writers have written good stories with strong plots without using any certain story structure formula. You may have taken some classes or read enough stories to recognize some basic steps. Plot beginnings will include something that happens to make your character want something she doesn't have. This is her first goal. Keep this in mind and start writing. If you later decide that the scene where the boy buys the horse is not the best beginning, you can fix or cut that later. You will have both learned backstory that may be worth weaving into the novel, and practiced your writing skills.
When I wrote Secrets of the King's Daughter, I started with a scene that was vivid in my mind--the scene where King Lamoni's daughter learns she was offered as a wife to "an enemy", Ammon the Nephite. For a long time I thought it was my beginning chapter, but no. New ideas developed and I learned what my plot needed after figuring more things out. I didn't have to toss it, just insert earlier scenes.
In summary, start with the pieces that you imagine and begin writing them down, first as notes and then as scenes. My next post will consider the end of the story. Part three will discuss middles. Until then, happy writing!