There are many levels to what a person must do to be considered a good citizen.
At the most basic level, the person must obey the laws. Good citizens pay their taxes. Good citizens obviously do not break the law in major ways like theft and murder. But a truly good citizen also does not break the law in minor ways that hurt the community, like dumping oil in storm drains or things like that.
At a second level, good citizens participate in their government. They must at least vote on a regular basis. Preferably, they will also make their opinions known on important public issues.
Finally, truly good citizens work to improve their communities. They do things like volunteering in ways that are beneficial to their community. They may help at their children's school or they may participate in church activities that are aimed at charity or community improvement.
In these ways, a good citizen obeys the laws and also participates in improving his or her community.
Being a citizen in the community depends on what country you live in, but mainly consists of contributing towards making the community a better place and exercising your rights and responsibilities as a citizen (voting, following laws, possibly contributing towards the armed forces, paying taxes, and attending school).