hmmmm roof chops can be tricky to pull off, alot of hte time people tend to want to chop off way too much roof and then it makes it looks fake and more than likely the pillars dont line up
in all realness, the roof chop in PS is actually a roof shrink
it laso depends on the angle and whatnot of the car, but prolly the simplest way is to select teh roof and all the background from the base of the roof upwards, then copy and paste the selection into a new layer, in the new leyer go to edit>free transform and then shrink the imaghe down till you get the roof as low as you want.
hopefully that works, if not then youll ahev to re-align the pillars with the body of the car, which can be really tricky, and is more fiddling around then following a tut as all pics are different and must be handled differently.
i hope this crash course helps ya out a bit if ya need anymore help just ask aight :)