I'm not sure exactly what you mean by "the first idea to provoke the story," but I'm going to assume you are asking what idea helps to set the story in motion. That comes out right in the first sentence of the book when Austen says,
"It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife."
In this time period, women were limited in their options. Few job opportunities were available to them, and most could not inherit their family's wealth. That went to the male relatives. So, finding a suitable marriage partner became a primary goal, and if he was rich, all the better.
Austen uses humor in this first sentence to set the story in motion. The wealthy Mr. Bingley has just moved into the neighborhood, and the neighborhood is abuzz with the news. Mrs. Bennett is anxious for her husband to go to meet him, hoping that perhaps one of her daughters will have the chance to become Mrs. Bingley. Other wives expect their husbands to do the same, with hopes for their own daughters.
The "truth" that Austen presents - that a rich, single man must be looking for a wife - is, of course, not necessarily true, but is the dearest hope of all the mothers and daughters nearby. Getting her five daughters married is Mrs. Bennett's chief aim, and much of the novel is taken up with the stories of how the eldest three find husbands. Thus, this opening sentence perfectly serves to set the story in motion.