Theabove is of course only half true - there's a lot we can do if that's what we choose. An easy start is to make the following your start page for your browser:
This site gives donations of cups of food from its sponsors according to how many people visit the site. There's a pointer there also to a similar site for donations of parts of tropical rainforest, research into breast cancer, etc.
The number of ways in which we can "save" our planet is vast, from simply visiting the above "Hunger" site once a day, through joining pressure groups such as Friends of the Earth and the Women's Environmental Network to going on marches in Prague or Seattle when organizations such as the World Trade Organization meet, or even writing to your MP. You can recycle your waste - take your bottles to the bottle bank, or to the supermarket where you bought them (in Hungary, you pay a small deposit on many bottles which you can recoup by taking the bottles back). You can use recycled paper toilet rolls and kitchen rolls. You can save electricity by turning your TV or your printer off when you're not using them because all those tiny lights consume power. We're living in a flat where lots of the light switches have a little red light in them when they're in the off position - I'm sure they don't cost me very much but it's another fairly unnecessary use of electricity.
An important aspect of caring for our planet is to care for the people on it. This can be done by building a sense of community around you, and it can also be done by reducing the level of fear which cuts us off from each other and from our surroundings. One way to do this is to talk to other people - it sounds simple but is actually quite hard, especially if those other people are from a different culture or speak a different language.
Servas is an international voluntary peace organisation comprised of a network of hosts and travellers who aim is to promote the building of peace and understanding by providing opportunities for personal contact between people of different countries, cultures and backgrounds. Have a look at their international site for more information. They also have a mailing list (ServasNet) which is open to Servas members and friends.
Of course, there are also pressure groups whose goal is to stop wars, e.g., Campaign for Nuclear Disarmament and the Hague Appeal for Peace. I suggest that stopping wars is more important than all of the above, although it all helps.