There are tons of things that people can do to help the environment, and some of them are very easy. Did you know that keeping your refrigerator and freezer full can help the environment? (It's true. Solids retain temperature better than air, so your fridge or freezer works less to keep a constant temperature). Buying local products helps too, as less energy is used in transportation. Buying things in bulk (less packaging) and buying produce that isn't prepackaged is a big help too (less material usage and less waste). Turning your thermostat down just one degree is a big help too (and saves you money).
More things you can do:
Carpool or take public transportation
Practice "trip-linking" - plan out your errands, shopping, work commute, appointments, etc. and make just one trip
Buy organic and fair trade products whenever possible
Donate your time or money to your favorite environmental causes
Use a laptop instead of a desktop (they use less power)
Install compact fluorescent light bulbs instead of halogens or incandescents
And here are some big things you can do:
Install solar panels or wind power. Use a grid-tied system and your excess energy production will power other buildings.
Build a home using renewable resources (straw bale, cordwood, papercrete, FSC certified wood, etc.)
Buy a hybrid, or a diesel (and run it on biodiesel)
Telecommute or start your own (green) business