You can change the world in a lot of ways, and some of them are as simple as holding the door for an elderly person with a walker or recycling your soda cans.
If you're looking for ways to make a bigger difference, here are some of the causes I like to support:
Sometimes it is lupus, despite what you hear on House. I know -- my mom has lupus, and has been living with the nasty, inconvenient, often life-threatening effects of it for more than thirty years. Supporting research into lupus -- which has a few different forms, but generally affects far more young women than any other demographic, is something I love to encourage anyone to do. You can learn more about lupus at The Lupus Foundation of America. Also check out itsnotsosexy, a very honest, and often very funny, blog/tumblr written by Maurissa Tancharoen Whedon.
Heifer International is a great cause. You can buy a goat in someone's name (or a flock of baby chicks, or an actual heifer!), and a real animal is given to a family somewhere in the world where need is great.
Everybody buys gifts for someone, right? Why not buy a gift that makes a difference? Novica, in association with National Geographic, provides artisans around the world a global platform for their wares. This means artisans in struggling parts of the world can support themselves and their families with their work.
Everybody needs to eat, too, and there are too many hungry children right here in the U.S. Feeding America works through a nationwide network of food banks and programs to provide nutrition to the 1 in 6 Americans struggling with hunger. That's far too many people in what's always been called "the land of plenty."
I'm a writer, so you can probably guess I feel pretty strongly about literacy and what reading can do for everyone. RIF and First Book are two programs that help to get books into the hands of children who need them, and right now there's only one book for every three hundred children living in poverty.
You can also Do Something! Their website helps you find a cause to work for, and suggestions for things you can do to help, even if it's just writing letters.