One very important quote (separated by a paragraph) comes right on the first page of the book. Huck Finn has become wealthy because of his adventures with Tom Sawyer in the previous book, and so he is taken in by the Widow Douglas as her son; she attempts to civilize him, and he finds it so restrictive that he runs away again. Tom convinces him to return because of a "band of robbers" (really just a group of boys playing) that requires their members to be "respectable."
I got into my old rags and my sugar-hogshead again, and was free and satisfied.
She put me in them new clothes again, and I couldn't do nothing but sweat and sweat, and feel all cramped up.
(Twain, The Adventures of Huckleberry Finn, gutenberg.org)
Here, Huck's character is defined very explicitly: he is a "free soul" and doesn't like to be confined either to everyday clothing or to the rules and restrictions of civilization. However, he is young and can't be trusted by society to make his own decisions, and so if he wants to live there, he must abide; even the clothing makes him feel "cramped up," because he is used to loose rags for comfort, not modesty. This is part of the reason that Huck eventually just goes off on his own, escaping the restrictions of society in order to make his own life and his own destiny.
There are other quotes in the links below, and the novel is full of meaningful quotes, phrases, aphorisms, and time-appropriate language.