...I don't know what that even means. Or do you just mean, "we already have too many people, the population needs to go down"?
Probably the biggest thing you could do, I'd say, would be to politically support pro-environment measures. There's a lot of "yes, but..." Americans who only support 'green' regulations and whatnot when it will not cause the slightest inconvenience to them. E.g.
"Yes I support bike lanes, but this +0.25% sales tax increase to build them is just too much, taxes are already too high."
"Yes I support mass transit, but I don't support redirecting any money from roads for cars to them because I drive a lot."
"Yes I support higher density housing, but I don't want to hurt my neighborhood's character."
You get the picture. From a practical perspective, America would easily make strong progress on reduced energy consumption, but the political will there is weak.