Firstly, I would make adherence to the precautionary principle a statutory requirement in decisions concerning the environment.
From wiki:
Secondly, massive investment into public transport in cities. Once there is a semblance of a transit system in place (with the mode of transport depending on the city, bus/tram/subway, driven vs driverless etc), I would implement congestion charges in inner cities (fee's to drive personal vehicles in certain areas at certain times) which would reduce traffic (less cars sitting around in the city) and would raise money to further extend and maintain the transit system. Exemptions to the congestion charge could be made in specific circumstance (lack of ability to pay coupled with lack of access to public transport or something - although efforts would be made to make access to public transport as far reaching as possible). As traffic flow lessens, pedestrianize certain streets/zones of the city.
Thirdly, a focus on enabling more renewable energy over fossil fuels. This includes nuclear. Nuclear energy is incredibly important in reducing our dependence on fossil fuels. Classic renewables (solar/wind/hydro) are brilliant, but are too intermittent to provide for America fully. Renewable energy would be used where it is viable (which is in a surprising amount of places). Govt grants would be given to companies pursuing battery technology, with the caveat that once breakthroughs are made, that battery technology is used on the energy grid, or if not, patents made available to those that would use it on the energy grid. Companies would be free to use said battery technology in other enterprise how they wish, which would be incredibly profitable for them, given our need to either increase the efficiency of lithium ion batteries, or replace them all together.
Fourthly, reduce all funding towards increased fossil fuel infrastructure. This doesn't mean ban oil or coal, but it does mean no more pipelines or factories.
And finally, as you inevitably critique my plan (it's by no means perfect, I didn't have any of this planned out before I wrote this post) you'll recall I put climate change as my number 1 issue. That means it becomes before economy.