I would build a city which would have very good organization of public transport, I would build a tram or if the city was large build underground train system. Every building there should be be built with ecological materials and have it’s own solar power heating or geothermal heating. The city would have strict law of recycling and saving water energy. Cars would be banned form the city area, a lot of cycling lanes would be build. Normal trains would be used as a means of transport to other cities. in the suburbs there would be a wind energy power plant for public needs( houses would have their own power). There would be a lot of green areas, in schools teachers would teach kids how to behave to keep the environment clean.
Greetings from Slovenia :)