First, always keep your manuscript saved in multiple locations. This could be on your computer, an external hard drive, and in cloud storage. Second, consider using watermarks or digital signatures to mark it as yours. And finally, don't share it widely until you're ready to publish and have taken the necessary legal steps to protect your rights.
It depends. If you're submitting it for traditional publishing, often double spacing is preferred. But for some digital platforms or self-publishing, single spacing might be acceptable.
Well, usually, you start with the title of the novel in a large and bold font at the top. Then, below that, include your name and contact information. Keep it simple and clean.