OK best place to get tickets is direct from the ACO in France, they run the event. The website is
www.lemans.org. From there you want to choose to see it in English and look for the option to buy tickets. I'd put a link up but I can't from my phone. You may then need to choose english again, it happens sometimes. There will be a few 'stages' at the top, the first being general admitance tickets. I really wouldn't bother buying these in advance unless you want grandstand seats. I would go straight to stage 3 which is your camping. Choose the right event, and then choose the campsite you want. To buy the tickets you need to register and when I did this is is all in French so have Google open to translate (don't worry, I don't speak a word of French so you know it's easy). Pay and your done. Then just book any ferry you want. I can guide you through on the phone if need be.
We always go on the Tuesday, turn up at around 8.30pm, I reckon a first timer would take a fair while longer as you have to navigate through Rouen and Le Mans. Rouen is a bastard but we know our way round it now. Le Mans is a lot easier to navigate.
For your money in the camping you get 35sqm which is enough for my 5 man tent and my car. A lot of sites have decent toilets (take your own bog roll) and showers (queues are hideous all morning meaning I rarely shower at Le Mans).
Any questions?