I often started my book at the wrong place... but I also often found the right place after I'd done revising it a few times....
The things to look for are:
What is your MAIN story?
A lot of writers, me included, build a whole lot of stuff around their stories -- backgrounds, character sketches, etc. that they think is also part of the story. Maybe so, but background stuff can be worked into the main story. Ask yourself, what is your MAIN story? Did you start the book three days before the MAIN story start? Did you start the book when the character was born? When is your start in relation to your MAIN story? The closer it is, the better.
Now we know the main story...
What moves the main story forward?
A lot of writers, me included, don't quite know. The back stories sound important... oh, we must show the ordinary life of the character before he makes the jump... don't we need to introduce our major characters? What about world building? All that stuff needs to be front-loaded, right? It takes some practice and objective eyes (and comparing your own work with other published novels) to realize, no, these are just background stuff... stuff that tells the readers: "Um, the writer is not ready yet; he's still finding his stride..." The literary equivalence of "clearing one's throat."
There are two important terms to understand: a) Inciting Incident, and b) The Point of No Return.
An inciting incident is an event that triggers the main story/plot. It's the ignition that turns the engine. It's the sparks that start the fire.
The point of no return is the door that slams shut behind the character(s). No exit. No re-entry. The tornado that takes Dorothy to Oz.
The PoNR and II can be the same thing, but they don't have to be.
I usually find my start when I focus on the "point of no return" -- that's when the character takes a leap and cannot go back to where he was -- either physically or psychologically, preferably both. I usually start the story just before that (some people would say -- plunge your readers in the first conflict, head first! In media res. That certainly is another approach... but the idea is to start the story as close as the point of no return, or at least the inciting incident that says, "Toto, we're not in Kansas anymore.")
Personally, I'm rather fond of the II that shortly precedes the PoNR.