What idea does Creon preach?

The play Antigone is really about the tension between the oikos (family household) and the polis (the city-state). Antigone, in wanting to honor her brother (whom Creon has labeled a traitor), speaks on behalf of family honor—the idea that the love and respect of family members should come before one's duty to the government.
Creon, however, refutes this view. Instead, everything he says reflects his celebration of the polis, even if it means putting his duty as a king before his duty as an uncle or father. In Creon's first public speech, he states,

"a man who thinks more highly of a friend than of his country, well, he means nothing to me...for I know well our country is a ship which keeps us safe, and only when it sails its proper course do we make friends."

For Creon, loyalty to one's country is most important, for without it, there would be no order or security.
It is important to note that Creon's lines are all delivered in public, so we cannot be sure what he really believes. He is often seen as a stubborn, blind, and raging fool who is cruel towards his rebellious niece; however, it's important to consider his position. He is terrified of anarchy--he witnessed Oedipus's demise, saw Eteocles and Polynices attack each other in a bloody battle, and now he feels enormous pressure as king to restore peace to Thebes. He must present a strong front, and preaching the importance of loyalty to the city is his way of establishing order. Of course, like in most Greek tragedies, his tragic flaw (hamartia) is his stubbornness. While we may see the core of his belief in the importance of citizens being loyal to their city as admirable, his refusal to consider Antigone's (or Ismene's, or Tiresias's) views leads to his downfall. 

In the play Antigone, Creon chooses to bury Eteocles with full military honors, while his brother's corpse lays decaying in an open field where carrion can feast on it. Creon decrees that anyone who touches or prays for Polyneices will be considered an enemy of the State and sentenced to death. Creon is only interested in retaining his power and maintaining social order. He refers to his enemies as "stiff-necked anarchists," and preaches the importance of following laws and decrees handed down by political officials. In Creon's opinion, individuals must respect the State regardless of their personal beliefs or grievances. In contrast, Antigone believes that citizens must follow the wishes of the gods before listening to politicians. Creon's refusal to compromise and view the situation from Antigone's perspective is a drastic miscalculation that has disastrous results. His insistence that Antigone's crime must not go unpunished results in bloodshed and hardship.