Research the text's period of history to understand how it participates in a socio-cultural context. If you are reading "Huckleberry Finn," for example, you might discover how the novel intervenes in debates at the time about slavery. Begin by searching for academic articles about Twain and slavery, focusing on those about historical conditions at the time. Also read or look for summaries of other works of American fiction around this time that also raise the question of slavery.
Take careful notes as you read your novel, noting all instances of whatever sociological phenomena you are trying to uncover. Using the "Huck Finn" example, think about the novel's representations of the slave Jim and the novel's tone in regard to him. Is the novel inviting us to sympathize with Jim's plight or to view him as less than human? Consider the irony in Twain's depiction of Jim and how radical or transgressive this attitude might have been at the time.
You can also read a novel from a sociological viewpoint without historical context. To do this, consider how it supports or critiques social institutions, often without knowing it, and regardless of the time in which it was written. Even a seemingly apolitical book, such as the "Harry Potter" series, can be read from a sociological viewpoint. Begin by considering the socio-cultural milieus in which it participates: the educational system, the development of children into adolescents and the social positioning of witchcraft, for instance.
As you read, keep track of how the book supports or undermines these social institutions. Is the "Harry Potter" series showing that boys develop into proper young men only when they are taken away from their natural families, or is it suggesting that Harry's lack of family is his fatal flaw? Do the books have any relevance for contemporary considerations of the American educational system? You might think about how school is less a place of learning for Harry than a place to form friendships.