I would do alot of stuff:

I would satisfy the worlds energy-demands by building solar power plants and nuclear power plants in space. The energy would be sendt to earth via a giant microwave transmitter, which would send a giant microwave beam to a reciever in sahara (so the beam dont fry us if it misses its reciever).

I would remove all carbon dioxide emmissions (and other dangerous gases) and only make electric or fuel-cell cars legal.

Prisons would be drasticly changed. Death penalties would be removed, and instead of being in a cell all the time, the inmates would work on large, ecologiacal farms (kinda like the scene from Sicko). That, along with banning bio-dieselmade from corn, would probably reduce the starvation in developing countries.

The school system would be changet to something similar to waldorf or steiner schools (minus certain things). High school would be mandatory.

Health care would be free.

With all the worlds resources joined, i would start large science projects to cure cancer, cloning and other stuff. I would also make a giant space program and try to make bases on the moon and a larger space station.

I would also beginto make a huge metropolis in the middle of the atlantic. The metropolis would be self-sufficient.
I would begin to make a space lift in the middle of the city.

I would probably do more stuff as well, but i honestly dont bother to write it all.