Think back to a period of change in your life. Can you trace that change back to one decision you made, or one thing that happened to you?
The inciting incident is the thing that happens to your character, without which there would be no reason to tell their story. If the inciting incident doesn't happen, your book doesn't happen.
For readers to be invested in your plot, the inciting incident must happen as close to the beginning of the book as you can get it. This is why people often tell writers to get rid of their prologues. Unless it contributes in some way to the inciting incident, it's taking up valuable space in the beginning of your book.
I analyzed Finding Nemo last week. Go ahead and read it if you need a refresher on the plot. I can wait. :0)
Finding Nemo opens with the scene of Marlin and Coral choosing their new home for their babies, right before they get attacked by a bigger fish. Coral and all of the babies die, except for Nemo. Marlin promises to take care of Nemo.
This is a prologue.
The next scene jumps forward in time to show us Nemo's first day of school. Marlin has finally decided to relax his grip on Nemo enough to allow him to go to school with the other fish children.
This is the inciting incident. If Marlin doesn't take Nemo to school, then Nemo doesn't get dared to touch the boat, and then doesn't get scooped up by the dentist, and then a generation of children doesn't get introduced to the absolute delight that is Ellen DeGeneres.
In the case of Finding Nemo, the prologue works because 1) it's short and 2) it establishes why the inciting incident is such a big deal. Instead of Marlin being a fish who just takes his kid to school like it's any old day, Marlin is wacked out about the prospect of Nemo being away from him. The prologue sets up the tension of the inciting incident perfectly.
Have you ever seen an agent refer to a MS "starting in the wrong place?" This often signifies a problem with the timing of the inciting incident. Here's another example:
I recently read the first two chapters of a friend of my husband's work-in-progress. The writing was good; very descriptive, with decent pacing. I had a good sense of who the main character was. But the lack of inciting incident meant that after those two chapters I had no idea what the book was about. And that's a problem. I mentioned this to my fellow writer, suggesting that he keep what he had written (because it was good writing), but move it elsewhere in the book so he could get to the inciting incident quicker.
And that's my advice to you: Evaluate your first chapter. Does the event which propels your entire story happen in that chapter? If not, you may need to shift some things around so that it does.