I wish I had enough money to be able to have a measurable impact on the world.
Subscribe to companies that have positive ethics, do what you can to extend their sphere of influence. It doesn't take much to make a difference. As time goes on and you have the means to be more influential both in terms of your audience and in terms of your finances use that to further extend the sphere of influence...
I realise that many people like their status quo, with nobody really paying them any attention. They have a comfortable life, they don't rock the boat too much, they play by the rules... or at least don't leave any tangible evidence of not playing by the rules... or bribe the right people to look the other way. But I think it's every person's responsibility to stand up and vote for what they believe in. People abstaining from voting and not standing up and rocking the boat when it needs to be done are part of the problem, not part of the solution. If people want to effect change, they've got to participate, and if you don't have enough people participating to challenge the status quo, then nothing will change, ever.
It is every citizen's responsibility to do what it takes to be informed and actively make informed decisions that will lead humanity forward.
Oh, you meant have a positive impact on the world. Well, that might cost a bit more, but you can put in some sweat equity by helping existing organizations who share your interests and values. Like the song says, we're all doin' what we can.