Who first discovered United States of America?

The question is flawed in premise. The United States was not a country until 1776. So if the question is then who discovered what is now known as North America then the question needs clarification. Do you consider the first people to settle the land as those who discovered it? If so than most scholars believe that the Americas were first settled by people of Asian desent between 4000 and 6000 years ago. These are the original desendants of the Native Americans including Noth and South American indeginous tribes. 
If you are asking about the first Europeans to set foot in the area then the answer is traditionally accepted as Leif Erickson. I believe he landed in what is now known as Newfoundland at the begining of the 11th Century. Erickson's travels did not lead to any permenat colonization. If your question is whose exploration voyages led to European colonization then the answer would be Columbus. But being your question was who first discovered the United States, awarding Columbus with the answer would not be 100% accurate because Columbus never set foot on any land that would now be part of the United States. While debated it is generally accepted that Columbus explored areas that are now the Bahamas, Cuba and Haiti. John and Sebastian Cabot both led expiditions that explored the northeastern section of what is now North America, but neither entered what is today U.S. territory. The first peron to sail to what is now the U.S. would be Spanish explorer Ponce de'Leon. He explored what is now S.E. Georgia and Florida. Hope that helps with your assignment.