Definitely start to care about someone other than yourself, something other than the things you have, and causes greater than yourself. That's where active citizenship starts.
Things you can tangible do to be good, more engaged, and an active citizen?
1) Read the news or watch the news. Then, make sure to come up with your own opinion and new ideas from what you have learned.
2) Volunteer for an organization you feel you can connect with, where it doesn't feel like work, but feels fun and inpsiring.
3) Exercise your right to vote (if you can). Even if you don't want to vote on every office or item, you do have the right to abstain on items. Make your voice count during election periods.
4) Instead of just advocating or being an ally, go out and do something. Act. Act with courage and believe that you can change the world of at least one person if you go out and do something.
5) Join AmeriCorps or the Peace Corps. These are very transformational experiences and teach you a lot about the US, the World, and yourself. The ultimate form of gaining perspective on communities around the world.
6) When you find a job, find ways to link it to social justice issues. Even if you are working for a large corporation/business, there are lots of ways to get involved in the community through the organization itself.
7) Donate (if you can) time and money to organizations you believe in.
8) Make small steps to become more environmentally conscious and friendly. Take shorter showers. Turn down the heat. Try to go without the AC. Do laundry in non-peak times. Use reusable, plates, bowls, utensils, cups, and water bottles. Buy a water filter.
9) Use social media to spread awareness on social justice issues.
10) Spread the word on all your amazing little and big acts of engagement and active citizenship. You might inspire to greatness.