If you just want a court to shoot around and have some fun with your 12 year old son, the slant surface shouldn't be too much of a problem.  I would say the most important thing is getting the backboard/hoop set up.  I would recommend an adjustable height style hoop.  As for the surface, put the hoop up at the lowest level.  The 17X36 concrete cement can be purchased at Home Depot or any similar store.  I would lay just a thin layer of cement, not too deep into the grount and let it dry.  Best of Luck.