Very few people start on page one and go right through to page 476.
Write what you have in mind now, then you can decide, later, where it belongs in the novel.
Look at some strategies of other writers and you will see that a novel doesn't necessarily start with the earliest chronological event in the story. Often, an intriguing piece of action opens the story and then flashes back to previous events.
Where you start may become apparent once you get into writing the story.
Figuring out some of the details and characterizations can be a first step. Many people start with an outline-- but nothing is set in cement. The most important part is to start writing and see where it leads you.