The most important thing you can do as a citizen is vote. It's easy to forget or feel disinterested and only vote during the really major elections, but it's very important to vote in every election you can. Running the country goes beyond which party is currently in power. You also need to vote for your direct representatives, who often play a much more major role in what changes happen in the country.
It's also important to vote in local elections. At the state and county level, you'll often vote on things like transportation initiatives, which can affect how people get to work, and other important laws and measures that affect your area including health and sanitation.