Before thinking about shading the walls I would suggest you go and look at some reference of real skyscrapers. There are not nearly enough windows and they should be closer together, a lot closer. They could also have some depth to them, not be flush on the side of the building.
The roof also needs details, ventilation shaft thingies, an access door to the roof, a somewhat raised border, and whatnot.
Don't get discouraged, you will get better with practise
