What are some important details of Joseph Andrews by Henry Fielding?

The way I understand your question is that you need to know the important details of what happens in Book I of Henry Fielding’s Joseph Andrews.
The narrator opens Book I by explaining that biographies are of value to society and by noting that virtue and purity are essential qualities.
The story begins in the second chapter, as ten-year-old Joey Andrews is working for the knight Sir Thomas Booby, failing to properly scare away animals because his voice is too sweet.
Joey sings beautifully in church and is an excellent student; he begins to learn Latin with the help of Abraham Adams, the church's curate.
However, the knight's wife, Lady Booby, takes Joey to London, where she tries to seduce him. Joey's morals are upright, though: even as gossip rages about him and Lady Booby, he refuses to be corrupted by her.
Sir Thomas Booby dies, and despite his wife being in mourning, she still tries to seduce Joseph. Seeking help, Joseph writes to Pamela, his sister, and on the way to mail his letter, he escapes from the advances of another woman, Mrs. Slipslop, who works in the Booby home.
Angry at being rejected, Mrs. Slipslop disparages Joseph, ultimately getting herself fired by Lady Booby.
It's clear that Joseph is handsome and charming, attracting attention from these women, and the narrator reminds us of this fact.
When Lady Booby tries again to seduce Joseph, he upholds his virtue again and mentions how his sister Pamela is very pure, also. Enraged, Lady Booby fires Joseph, then mends her relationship with Mrs. Slipslop, who now has power over Lady Booby. Joseph writes to Pamela again and, dismissed from his job, heads to his home in the country.
Back at home, where Joseph is heading, his real love is a girl named Fanny Goodwill; the two of them are eager to get married, but they are waiting until they are of a proper age.
As Joseph is traveling home, he endures bad weather and an attack by two violent thieves. He manages to get a ride on a stagecoach to an inn, where he rests; he's become very sick. He babbles feverishly about forgiving the thieves who beat him and giving up his lust for Fanny.
Parson Adams arrives at the inn, helping Joseph with support and money. It becomes clear that the surgeon tending Joseph is immoral, just like the thieves who attacked Joseph, and Adams borrows more money to help Joseph make his way home. Adams is eager to publish his sermons, so he returns to London and ensures that Joseph rests some more before attempting to travel again.
At the inn, trouble arises as one of the maids, Betty, is caught sleeping with the inn's married owner. Betty also tries to seduce Joseph, who spurns her advances, and soon Betty is fired.