Check out the LFS wiki (wiki.liveforspeed.net), as it has a helpful guide to LFS rendering. It can't possible teach you everything you need to know, but it's almost step-by-step (if you have Brazil), and will give you a chance to fiddle.
Fiddling is the only way to learn really, so don't be afraid to try things. The worst you can do is have to reload a previous save or cancel a render that says it'll take 15 years.