I think that is probably the question everybody asks at some point in their life.
I believe the hardest thing is to discover what is your mission in this world.
I mean this as a personal challenge.

Concernning the rest of the world, you can do lots of things to help your fellow man, to help nature, to save the world even if you do simple things like: treat others with respect, help your friends, family, community and other people in need; recycle, save resources like water, energy, polute the least as you possibly can; plant a tree, take care of gardens, find a home for an abandoned animal...

There are so many simple things you can do. And if all of us made a little effort, the world and our lives would be so much better.