I quite like the your last skyscraper...but!!! the base seems out of scale
to the rest of the building.I really think you need to find a building in real
life...lets say some 20 floored American office block.Google images would
be a good place to start.Pick an office block and then do google images.
Then try to recreate it in sketch up and if you do it well that will be
the basis for the rest of your models.Sometimes people jump to
do skyscrapers but really its more often harder to do a smaller building.
You really need to be strong on the basics to recreate such a building.
So find a building and recreate it to your best.It would be a test.
the interiors aint bad,the window aint too good.