In short, you would do well to familiarise yourself with two key areas before heading over. First the main one, the track layout itself. If you can use a games console great as its more interactive or even just study the many youtube videos that are around of cars heading round.
You wont get a true feel of the gradients or the variable surfaces that you encounter on the way round but knowing if you are on a fast section or a slow one would be very useful and knowing which way the next corner goes is invaluable, as most of them you can't see the exit on approach and so you are running blind as to where the apex sits.
Second on the preparation list would be to research the rules, etiquette and practices of the place. What makes sense here and there aren't the same. For instance if you are taking the racing line on a left hander and a 911 doing twice the speed collects you from behind and shunts the pair of you off the tarmac, that ones your fault for not being on the correct side of the road (the right). Where contact is made with another car the police tend to be called and where contact is made with the barriers, the credit card better be handy.
Ben Lovejoy's site which I've posted a link to is invaluable. Getting over the water gets cheaper the further south you go, the Hull ferry is a reasonable blend of driving hours vs hours on boats and cost. My preference is the Tunnel as its so flexible for crossings, takes no time at all and costs peanuts. It is a long slog home again though bearing in mind you have a 4 to 5 hours drive on the continent side to any of the ports.
Finally, keep a very watchful eye on everything you know about mechanically on your car, having made sure its up to the trip before you go.
We take spare brake pads, exhaust clamps/rubbers, all types of fluids, at least one full size spare wheel if not more, spare plugs & anything else we're aware of being a weak area on the cars concerned. Plus a couple of jacks and as many tools as we can carry & are capable of using. (the spare stuff comes out at the hotel though!)
The cars come off every lap or two usually to have the tyre pressures, fluid levels etc checked and generally left to cool off while the drivers do the same. Its particularly hard on brakes.
p.s. Respect the place and be humble, older slower cars will be faster than you. If something approaches from behind, hang right, indicate right and let them go, particularly if its a white BMW 3 series with another close behind!
