Every community is different with various expectations and norms. That said, there are some guidelines for being a "good citizen" that most anyone could agree on:
Some people contribute to their community through organized volunteer work. Some make the goal their actual career. And others simply find a way to really see those in need—from the person in a wheelchair who can't grab the cereal off the top shelf at the grocery store to the child on the playground who can't seem to find her mom—and jump in to help. This takes being less self-absorbed and more conscious of the needs of others, but it's vital to being a good citizen and making a community as efficient as possible.
Everyone in a community has unique talents, and it is through the combination of those talents that the community can do well. Not everyone has the same abilities, and being able to work hard with given abilities also makes someone a good citizen.
Be a Voice in the Governing Process
Everyone over 18 can vote, but it's also important to keep abreast of issues facing the local community and to to be a voice for change when and where it's needed.
This means that good citizens don't commit crimes against their fellow community members, and it also means that good citizens don't slander or otherwise disrespect community members.
Make the Community Better
There is a saying that provides solid advice here: When you arrive at a place, leave it better than you found it. This prevents simply being complacent in maintaining a status quo. Good citizens look at their communities and determine not simply how to maintain it but how to make it better.