Christopher Columbus (1451-1506) was an Italian explorer, most famously remembered for his accidental discovery of America. For possible essay questions, you could consider Columbus's attempts to gain patronage through his great 'enterprise of the Indies,' particularly his relationship with the French, English and Spanish courts. You could also look at the voyage itself: what was life at sea like for Columbus and his three fleets? Once he had landed in America, how did he interact with the natives? What were these encounters like?
What about other discoveries of America? You could write your essay with reference to the Viking explorers who landed in America before Columbus and compare and contrast the two discoveries.
Finally, consider the wider consequences of Columbus's discovery. How did the world react? What happened to Columbus after he discovered America, or what is his historical contribution and significance?
Hope this helps.