Well, pencil and paper practice is always helpful, but in this case, i think the design is ok, the office looks nice. You mainly need to work on your colors. They are all too saturated, sometimes with too much contrast and other times witrh too little, and don't combine very well with each other, which forms an amateur and unrealistic look. I suggest playing with those color properties until it looks a bit better (get a program with good palette support, where you can just pick a color and play with sliders and see the pixels on the piece change immediately).