I live 2 hours from the ring and have done quite a few laps and nearly stuffed a few cars, here are my tips
1)Flush your brake fluid! if your brake fluid is more than a few months old flush it and put some high quality fluid in. As a beginner you will be on your brakes alot! You Will Over heat them and they will start to fade. if you dont chage your fluid it will boil and they will just dissapear, normally with a very bad result.
2)Don't let your ego write checks your body can't cash. There are locals there who do laps daily, You WILL be passed by one of them in a tatty looking old Golf or e30 shape BMW. Try to follow or stay ahead of him and it will end in tears. Skill and knowledge of the track mean more than BHP on this track.
3) If you think you are doing great you need to slow down. Most people get thier first lap out of the way fine so relax. Statistically Your third lap is the most dangerous because you have more confidance and knowledge of the track and you will push past your limits.
3)ADAC is the German AA, I am covered by them for breakdowns no matter what country I'm in for 90 Euro a year. it is very expensive to be towed off the track or the Autobahn. If you live in the UK you have to call as the system will not accept an address outside of Germany without an operator)
4) Some Rad weld can be a lifesaver if your 15 to 20 year old radiator cracks an endtank while pusshing extended high speeds on the autobahn/ ring
5) spare oil. A liter of oil will cost you 14 euro a liter at a petrol station for cheap crap.
6) A full size spare as has been suggested is a smart idea.
7)Cash, lots of. if you are caught doing something naughty by the police on a foriegn plate they can demand the fine on the spot and if you are unable to pay they will impound your car.
You will find the adrenaline addictive, as well as the beer bratwurst and Unlimited Autobahn. If you are sensible it will be a trip you will never forget, If you are not sensible it will also be a trip you will never forget but wish you could.