Honestly, I don't there is much that one person can do change the world. I like to think realistically, even though people usually see that as pessimism. Think about the people throughout history that have "made a difference". Has their impact reverberated through time, on a global level? Did they stop us from wanting to hurt and kill each other?
I just try to change things on a small scale. I try to help my friends out when I can. I pick up a piece of trash if I see it lying on the ground. I turn off lights and appliances when I'm not using them. Sure, they are small steps, and don't impact the world in the grand scheme; but they are tangible things that I can do to make my life, and the lives of those around me, a little bit better.
I just try to change things on a small scale. I try to help my friends out when I can. I pick up a piece of trash if I see it lying on the ground. I turn off lights and appliances when I'm not using them. Sure, they are small steps, and don't impact the world in the grand scheme; but they are tangible things that I can do to make my life, and the lives of those around me, a little bit better.