This is a great question because I decided to write a novel last year and I had this same problem. I realized that sometimes you don't know how to start. You can't go with "once upon a time". it's too walt Disney and it's childish.
The best beginnings start mid-moment. They start in the middle of a conversation or with someone looking out the window of a house or with a person thinking.
For example, my book starts with a boy going to an academy. I didn't know whether to start with his waking up or him already at the school. The first would require unnecessary details and the second would skip too many. So I started with him walking to the school with his mom. To paint a better picture of the plot, I had them reminisce a bit to give an idea of their background and why he was going to the academy in the first place. Then when I was satisfied with it, their conversation changed to them talking about what would lie ahead. So I guess what I'm trying to say is that the best beginnings aren't exactly at the beginning, but close to it. Close enough to give a mysterious background but not so close as to paint a definite picture of the character. there has to be something for the readers to learn as they go forward.