Many of us have heard this cardinal rule: don't use prologues. But we all know there are exceptions, right? When we want to start our story with a prologue, I think we tend to convince ourselves that we're one of the exceptions.
So how can you tell? Agent Kristin from Nelson Literary Agency posted Why Prologues Often Don't Work. Kristin doesn't pull punches and isn't afraid to tell you exactly what's wrong with your prologue. Odds are, you should cut it altogether. (I know, it's heartbreaking.)
Once the prologue is out of the way, you can get to your main story. But wait. There are some cardinal rules about regular beginnings, too. Kidlit tells you how to Fix Your Beginning by giving a list of things to avoid.
Beginnings need to be razor-sharp and brief. The Blood-Red Pencil gives details on this in Part 1 of their series on Big Edits.
Don't let these scare you out of starting a novel, though. Worst-case scenario: you use one of the forbidden beginnings and you'll need to edit it later. :)