In terms of rules and regulations, there are none. When I went a couple of years ago they didn't check anything on the car (or license/insurance) - just turn up, pay and drive. Worth reading up on and watching the videos at the track about rules and safety whilst driving.
I'm going again in the Celica in just under a month. I'm not planning on doing anything special to the car, just checking tyre conditions and pressures, make sure the car is running smooth (service), making sure all loose objects are fastened down or removed. Warning triangle, high-vis, fire extinguisher e.t.c. are all a good idea, as well as a list of Nurburgring emergency contact numbers.
I'm not planning on pushing the car hard as the Celica is too valuable to me and I don't want to go beyond its limits into the armco (thats until the red-mist kicks in! .) The best preparation you could do is learn the track (and your car) as best you can. You'll never get the impression of how bumpy or steep the gradients are, but learning the layout, black spots and best lines will help massively. There are a few tight corners hidden which you won't see until too late - the weakest point in most cars is the driver.