Novels tend to be more difficult to write than nonfiction because novels create worlds and characters that have to make sense and be consistent. This is especially important for those who write fantasy or sci-fi. Professional editors cost money, so best to first try a little self-editing on your own to make their job easier and hopefully cost you less.

The Alliance of Independent Authors has a post about how you can work on self-editing your fiction. The post is a bit complex, but you can glean important tips:

How to Self-Edit Your Novel: What You Can Learn From Professional Editors

(Always get a professional editor after you think you have your final draft, and prior to that if you need help developing the story)