A good introductory paragraph establishes a contract with the reader. In it the student states his hypothesis and shows how he plans to defend it. 

A good introductory paragraph shows the reader a blueprint of how the writer will construct the body of his argument.

The Thesis Statement (Darling et al)
Introductory Paragraphs (Darling et al)
Paragraph Development and Topic Sentences (Darling et al.)
The Five Paragraph Essay (Darling et al)
Writing About Literature (Darling et al)

The Topic Sentence (Strunk and White)

Paragraphs and Paragraphing  (Purdue OWL)
Paragraphs (Length Considerations) (Purdue OWL)
Developing an Outline (Purdue OWL)

From a 10th grade student's recent essay on Conrad's Heart of Darkness, here is a superb introductory paragraph. Notice how the writer accomplishes the essential objectives in a good introduction:

           In his novel Heart of Darkness, Joseph Conrad reveals the terrible source of evil at the core of human behavior. Through the use of engaging symbols which employ the Freudian methods of free association and dream interpretation, Conrad leads the reader deeper and deeper into racism, deeper and deeper into the heart of man, deeper and deeper into the heart of darkness. At the very source of this darkness reside man’s repressed emotions and fears, the Freudian Id, and man has no greater fear than losing control over his own actions. Therefore, man suppresses his very nature, or Id, with the superego, or conscience, so that he can prevent his wish from becoming reality. Humans want to satisfy the Id so badly that they are terrified of it and of the monstrous transformations that would result if humans were freed to act on their most instinctive impulses.  In a cruel, ironic twist of fate, man fears what he desires most. This paradox is the secret root of racism. Racism is solely a love impulse so fearfully denied that hate occurs. In this way, the story is not about Africa or Africans, but all men. Racism stems from man’s fear of his own most basic nature.

Take the following topics and construct a good introductory paragraph to an essay on each of them:

The greatest baseball player of all time.
Abortion 
Income taxes
The Cause of the Civil War

In a good introductory paragraph two goals must be accomplished: 

A good introductory paragraph establishes a contract with the reader. In it the student states his hypothesis and shows how he plans to defend it. 

A good introductory paragraph shows the reader a blueprint of how the writer will construct the body of his argument.

The Thesis Statement (Darling et al)
Introductory Paragraphs (Darling et al)
Paragraph Development and Topic Sentences (Darling et al.)
The Five Paragraph Essay (Darling et al)
Writing About Literature (Darling et al)

The Topic Sentence (Strunk and White)

Paragraphs and Paragraphing  (Purdue OWL)
Paragraphs (Length Considerations) (Purdue OWL)
Developing an Outline (Purdue OWL)

In the example below a 10th grade humanities student is clarifying a psychoanalytic analysis of racism as expressed in Conrad's Heart of Darkness.

         Arriving at the inner station, Marlow finally meets Kurtz, the man who has lived solely to gratify the id.  Kurtz had raised an army of followers. Every native in the land around him worshiped him as a god: " We must necessarily appear to [the savages] in the nature of supernatural beings- we approach them with the might of a deity," wrote Kurtz (Conrad 2:50).  Even the Russian journalist, the only other white man living at the inner station, now worships Kurtz as a god and revels in Kurtz's willingness to do anything he pleases: 

Kurtz has demolished his superego and allowed his id to take complete control of him; in contrast,  the general manager only used the id when he wanted to: "Everything belonged to him- but that was a trifle. The thing was to know what he belonged to, how many powers of darkness claimed him for their own." (Conrad 2:49) Marlow criticizes the other boat travelers because they can’t understand Kurtz, because they are tamed by the superego and by civilization. Like Macbeth, Kurtz has transcended the restraints of morality and descended into a nightmare.  "The awakening of forgotten and brutal instincts, by the memory of gratified and monstrous passions. This alone, I was convinced, had driven him out to the edge of the forest." (Conrad 3:65). Kurtz is beyond conventional racism because he strives not just for domination but for absolute power, the power of a god. Yet when Kurtz is on his deathbed, his last words after the final vision of his self-indulgent life are "The horror! The horror!" (Conrad 3:68). Kurtz journeyed within himself and discovered in the id the most primitive and ugly level of barbarity, the terrible source of evil within us all .

The Topic Sentence (Strunk and White)

The Thesis Statement (Darling et al)
Introductory Paragraphs (Darling et al)
Paragraph Development and Topic Sentences (Darling et al.)
The Five Paragraph Essay (Darling et al)
Writing About Literature (Darling et al)

Paragraphs and Paragraphing  (Purdue OWL)
Paragraphs (Length Considerations) (Purdue OWL)
Developing an Outline (Purdue OWL)

The Topic Sentence (Strunk and White)

The Thesis Statement (Darling et al)
Introductory Paragraphs (Darling et al)
Paragraph Development and Topic Sentences (Darling et al.)
The Five Paragraph Essay (Darling et al)
Writing About Literature (Darling et al)

The Topic Sentence (Strunk and White)

Paragraphs and Paragraphing  (Purdue OWL)
Paragraphs (Length Considerations) (Purdue OWL)
Developing an Outline (Purdue OWL)

Concluding Paragraphs (Darling et al)

Here's a wonderful concluding paragraph to a recent essay on Voltaire's Candide:

Humans simply cannot control earthquakes, death, and all the evils of the world.  It is pure arrogance to assume that they can.  However, there is no need to endure the petty, private, deprecating actions of the church and the absolute monarchs.  The church and the aristocracy do not have the only right answer. Although man cannot control neither nature nor good and evil, he can control his government.  Man can stem the root of many social evils at their source: Man can separate the church from the state.  Man can depose absolute monarchs who enforce their will on others without regard to the common good.  Man can devise constitutions which abolish class and protect the rights of the individual Through inductive reasoning, through observation of the state of matters, humans can devise the best plan for the state.  Where the absolute monarchs used deductive reasoning and started from a truth that was in fact already false, the new monarchy will strive towards a truth based on the wisdom of many people. The new monarchy will come closer to good and the truth than anyone has come before.  However, it will be through choice. Man chooses his destiny by changing the government. Liebniz is wrong. The words of the great poet, Alexander Pope, are wrong. 

Concluding Paragraphs (Darling et al)