So, any long writing project is a marathon, not a sprint. It's an endurance run.
What I found useful were two tools when I was starting out:
Outlines - once I knew where the story was going to go and the steps to get there, it made it easier to write my way through them.
Dividing the story into bite-sized pieces. It's a lot easier to write a part with 8-10 chapters, and then another, and then another, than a full novel that is 30 chapters long.
As far as the content goes, the only person who knows that is you. You would probably be better served by trying to tell a smaller, more limited story just so that you can complete it (and know that completing a story is a thing you can do), but it should be a story that you want to tell and speaks to you. It has to be a story that you care about telling. Otherwise, it won't be as good as it could be.
So, ultimately, the answer to your question has to be another question. What should your first finished writing project be?