Identify and record what you could do in the future to contribute to citizenship in the local or wider community.
In my classroom, I teach about good communication skills, good choice making, respecting self and others, avoiding labels, stereotypes, and racism. I reinforces the positive and I correct the negative. These things I hope my students take home to their families and into the community. This is how I make a difference.
Most communities have many organizations that you can get involved in and volunteer your time to better your community. Also many school districts are starting to incorporate community service projects into their curriculum, at least in some subject areas.
Citizenship is such an important thing because it's what ensures that everyone has a stake in what happens in our neighborhoods, towns, cities, states, and country. The more involved one gets, the more connected he is to his community, large or small.
Some other ideas for community involvement include volunteering at a local animal shelter or animal rescue league, leading a volunteer crew (with the proper approval) to clean up a park, or offering to help at a nearby senior citizen center. I commend you for your interest in serving.
The current political season is an exciting time to participate in activities that promote good citizenship, not the least of which is voting in the upcoming elections in November. After the election, regardless of whether your chosen candidates were elected, you can continue to be politically involved by joining your local political party precinct, if you are registed with a party. Precincts provide many ways to support candidates, and not only during election season.
I assume when you say "contribute to citizenship" you mean ways that Americans can actively participate and become involved in the community as citizens. I am also going to assume that the participation ideas you seek will also naturally contribute to the greater good of the community.
Many high school students turn 18 during their senior year (some a little sooner). Arguably, the most important thing you can do when you turn 18 is register to vote. The next step of course, is to become an educated voter, inform yourself of the candidates and issues during election seasons, and go vote. The is the one right and priveledge of American citizens that is probably the most largely taken for granted, as evidenced by the general lack of voter turnout (especially in local "off-year" elections).
In addition to voting, there are several community outreach opportunities to become involved in as a member of a town, city, or even state. If you start locally, you can find several places to become plugged in, any of which can be found by searching for areas of interest. For example, if you are particularly interested in (or thankful for) the ability to use a local library for free, you could seek to become part of the "friends" of the library. I know for a fact that most public libraries have volunteer committees who meet to discuss extra funding opportunities, new/old materials, etc. Often these committees are made up of mostly older and retired people. They would welcome a young member to contribute for the voice of the younger generation.