A duty conveys a sense of moral commitment to someone or something.  This commitment results in action and is not simply a passive feeling.  It requires some sense of sacrifice of self-interest.

So, duty as an American citizen starts with a sense of pride, commitment, or love for your country.  That motivates a desire on your part to “return the favor;” to try to do something positive for your country. 

Expressing your individuality when honored to sing the National Anthem at a sporting event (had to mention that, it’s fun to watch these videos).

Running for elected office

Serving on a government committee

Working for a campaign or getting involved in an issue that impacts your community

Volunteering for service in the military

Working for a charitable or community organization to improve people’s lives

This is one of the reasons scouting places such a high value on volunteer activities.  The service project you must organize and accomplish to become an Eagle Scout will help you appreciate the difference you can make in your community.  Every eagle scout I have known has gained an enormous sense of pride in what he accomplishes, and you will too!  As you move on in life, use that same spirit and initiative to be an involved citizen – to do your duty as you best see fit.