Sorry just visiting this thread again since my last post.
50mm prime (fixed, i.e doexnt zoom) lens really is only good for close up people shots, they have a very shallow depth to them.
I'd recomend using another lens for car shots etc, the standard kit one or a upgraded version if you have one. My walkabout lens is a 17-70mm one, and I only use my prime for people stuff, anything with more than a heads worth of depth to it = I use the 17-70mm.
When you take the pic look and see what F/stop number it is taking it at, you can usually see this via the view finder, i.e 7.1, 11, 5.6 etc. Lower = less range in focus, higher number = more range in focus. With cars you want to be around 9-11 I'd say.
As for ISO quesiton, totally depends on the light and your F/stop, the lens etc, 'generally speaking' for the benefit of this thead....
ISO100 = perfectly lit sunny day, ideally with tripod
ISO200 = good light, i.e sunny
ISO400 = pretty good light maybe a bit cloudy
ISO800 = the light isnt great, either getting dark or its rather dull/little natural light
ISO1600= very little natural light
Higher the ISO the more grainey the pics will look, doubt you will notice this until you get to ISO800 though. Of course lower ISO= longer shutter opening to cpature the image = more chance of blur/camera shake (hence why people always say get a good tripod).
Hope this helps.