As my colleague states, Sam's actions constitute the most egregious examples of corruption in the novel. In the story, Sam is the president of the fictitious West African country of Kangan. His two friends, Christopher Oriko and Ikem Osodi serve as advisors in his cabinet. While Chris is the Commissioner of Information, Ikem is the editor of the state-sponsored newspaper, the National Gazette.
Although both Chris and Ikem support Sam, they are wary of his tendency to quash dissent of any sort. Due to his military training, Sam is characteristically imperious in nature, and as time progresses, he becomes unduly paranoid about his ability to hold on to power. Sam eventually decides that he should be elected "President-for-Life" and he proposes a people's referendum in support of this important step. However, to Sam's great dismay, his native province of Abazon refuses to support his pet referendum. The referendum fails, and Sam blames Chris and Ikem for the failure.
To punish Abazon, Sam orders all the water-bore holes in Abazon (holes that have been drilled for the purposes of extracting water) to be closed. Sam knows that there is a drought in Abazon, but he wants the people to feel the heat of his displeasure. He also refuses to visit Abazon when the Abazonians plead for his help to end the drought. Sam's arbitrary order to close the boreholes is one example of his corrupt behavior.
Next, Sam commands Chris to fire Ikem from his position as the editor of the National Gazette. Sam wants Ikem fired because he suspects that Ikem has colluded with Abazon agitators (derisively termed by Sam as malcontents who hold demonstrations in front of the presidential palace). When Chris balks at Sam's order, Sam threatens to implicate Chris for his possible role in defeating the President-For-Life referendum a couple of years ago. Meanwhile, Chris holds firm, and Sam works to fire the man himself. This is another example of Sam's corrupt behavior: he is willing to sever the bonds of friendship in order to protect his tenuous hold on power.
Later, Sam has Ikem executed because of a tasteless joke. Accordingly, rumor had it that the Central Bank of Kangan was planning to put the President's image on the nation's currency. When asked his opinion about this in a lecture he was giving, Ikem answered: "My view is that any serving President foolish enough to lay his head on a coin should know he is inciting people to take it off; the head I mean." The next day, the National Gazette's headlines read: "EX-EDITOR ADVOCATES REGICIDE!"
Even though Ikem never meant any real harm by his joke, he is nevertheless hauled off in the middle of the night by Sam's men. Thus, Ikem is executed not because he has planned an actual insurgency against Sam but because he represents a grave threat to Sam's power. This and other examples of Sam's abuse of power demonstrate his corrupt behavior in the novel.
The best place for you to start is with a character analysis of Sam. He is the major force of corruption and misuse of power in the novel. Sam is obviously someone who was untrained and unprepared for leadership before he became president, and now given a position of absolute power, he lets it go to his head. He insists that everyone calls him "Excellency" and wants to be President for life. However, he completely ignores that with power comes responsibility (the motto of Spiderman, incidentally) and is ignorant of the needs of his people and what is best for them. He relies almost exclusively on his cabinet whilst at the same time mocking them and belittling them. In the end of course, we see the abuse of power most clearly when Sam chooses to have his childhood friend, Ikem, killed, as a result of his own megalomania and paranoia. Achebe in this novel shows the danger of pursuing power for power's sake and also the impact of a dictator on a populace - the people of Kanga only suffer from Sam's leadership.