I think that the three most important duties of the United States Citizens are to vote, pay taxes, and to obey authority. These three duties are a necessity to the functioning of the country as a whole.
The United States only functions with the citizen's help. Voting supplies the government with an overall opinion of who the people want. Without the voters of America the officials would be appointed within themselves. The government would be all powerful. Democracy is supposed to lay the power with the people. If the officials appointed other officials and the people did not vote we would have no democracy.
Without paying taxes the government would not be able to employ its officials or pay for the roads we drive on and any other necessities that the government supplies. The tax dollars that we send away, without fail of complaints, pays for us to be able to have an orderly society with a court system, police protection, paved roads and other civil services.
Obeying authority is the most important of all of these responsibilities. When the laws that we vote on are broken the police come into action. These people are paid for by us too. We pay for these people to keep order and take criminals off of our streets. When there is a mass disobedience the police may not always be able to stop it. This causes the United States to look trashy to other countries. When the people listen to the government and take civil actions the other countries look at us and think "wow", democracy works well in the United States.
These three things help make the United States seem civilized and well working. When the citizens follow these rules they are likely to get ahead in life by learning...