Can anything you do really matter to the Earth? On a universal scale, no. The Earth will keep on spinning, evolving and sustaining life no matter what humans choose to do. There's much to do, however, to reduce the pressure on our Big Blue Marble's limited resources.
Consume less. Everything people do, including eating (especially meat), watering lawns, heating homes and driving cars, consumes resources. Everything people buy requires resources to produce and ship. Be conscious of all the small decisions you make in your everyday life that increase the total human impact on the planet. See related eHow articles How to Live With Less, How to Live Off the Land and How to End World Hunger.
Choose to have fewer (or no) children. The pressure on Earth's resources by its ever-increasing human population is one of the most dire issues that the planet faces.
Learn what types of fish are in danger of being overfished and don't buy them at the store or order them in restaurants. The United Nations site offers background information (see Additional Resources). For specific recommendations on which fish are caught and farmed in ways that support a healthy environment, check out Seafood Watch at the Monterey Bay Aquarium's site (in Resources).
Consider driving a smaller or hybrid car. Of course, buying a new car equals more consumption, so approach this issue with some caution. But if you're already looking for a new car, get one that's energy efficient.
Support international agreements to limit the output of greenhouse gases. Reduce your own energy use as much as possible. See the eHow titled How to Prepare for Skyrocketing Energy Costs.
Downsize your life. See Related eHows How to Get Organized and How to Get Rid of What You Don't Want.
Buy organic food. Pesticides take a toll on the environment and frequently spawn pesticide-resistant pests. Buying organic food directly from the growers supports small farms and promotes biodiversity. See the eHow on How to Prepare an Organic Vegetable Garden.
Lend your energy to protecting the drinking water supply on both micro and macro levels. Access to and availability of clean drinking water is a growing global crisis. Cut back on personal water use with water-wise gardens (How to Design a Dry Garden) and promote development of sound water policies in your town or region (How to Manage Growth in Your Community). Or join forces with scientists working to perfect the difficult and expensive process of desalinization (waterdesalination.com).