This portion of your story is called the "Trigger," or the "Inciting Incident," because it kicks off the action that sends your character out of the Normal and into the Unknown.
The Trigger is usually a change: someone important to the protagonist dies, the protagonist is accused of something or cursed or sent away, the protagonist inherits a plot-centric item, or hears about one. In any case, the protagonist is prompted by either internal or external motivation to leave the Comfort Zone for the Adventure.
As an example, let's take a look at Star Wars. Assuming Luke is our protagonist, the Trigger for his story is the purchase of R2-D2 and C3PO, because it's at that point that everything changes. Their presence and actions lead to the events that force him to leave his home, travel with reckless strangers, fight evil empires, and rescue princesses. (The golden age of sci-fi/fantasy, right there.)
For another example, we might look at Spirit; Stallion of the Cimarron. The Trigger for Spirit's story is seeing a campfire in the distance. His response to that leads directly into his adventures for the rest of the story (which include him leaving his home, rescuing a helpless human, and falling in love, among other things).
There's not really much more to this particular element. It's an event, meeting, ultimatum, challenge, or rite that forces the standard of life for the protagonist to change. And it doesn't have to be directly related to the rest of the plot, either.
In Spirit, for example, the cowboys whose fire Spirit sees at the beginning of the movie are not responsible for or even directly associated with the military outpost he's taken to, the railroad he's sold to, or the Native Americans he's stolen by.
Once your Inciting Incident has taken place, the story can begin in earnest, and there are adventures waiting to be had just up ahead.
*There is an exception to this rule: Slice of Life stories.
From my personal experience, Slice of Life is a genre that accurately depicts a section of a normal person's life, and it's often incredibly boring. At any rate, the French 1800's Slice of Life excerpts I read in college were among the most mind-numbingly uninteresting things I have ever read.
The Trigger is an event occurring to or near the main character that affects a change to the character's standard of living, either by forcefully removing them from the norm, encouraging them to abandon the norm, or prompting them to save the norm. Take your pick (or mix and match).
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