Citizens can influence what the United States government does through four channels: political parties, campaigns and elections, interest groups and the media. Collectively, they're called linkage institutions as they "link" citizens to the government.
The Democratic Party and Republican Party, the two major political parties in the U.S., and minor political parties each represent ideologies that people can align themselves with and thereby link themselves to government through. If the party's point of view, as expressed through its public policies, are consistent more or less with the citizen's, then that party usually earns the citizen's allegiance.
Party members are directly elected by the citizens. Campaigns and elections, which can be lavish, expensive affairs, afford people the opportunity to hear from party representatives regarding issues and a candidate's qualifications for office. The information citizens rely on to elect their government officials generally comes from the media, such as television, radio, newspapers and the Internet. Citizens share their opinions with their elected officials through various ways, including letter writing.
Special interest groups center on subsets of people with common interests and attitudes who try to influence government to share their philosophy. Generally standing for one issue in particular, they may rally around their concerns according to profession, business, corporation or hobby.