Saj
Visit
www.beermountain.com - tells you everything you need to know. There are a few other advisory sites as well.
Assuming you would be camping, you can actually do it without any pre-booking, i.e. turn up at the channel tunnel, wait for a slot - ge to Le Mans & drive around until you find a campsite. But, with this approach you need to get to Le Mans at least 24 hours (best 48 hours) before the race starts. If you want to arrive the evening before or on the day, pre-booking is essential.
If you want to be anywhere near the grandstand you have to get tickets and be there well in advance of the race. Worth it for the pre-race antics!
When choosing a campsite you have to consider how close it is to the circuit: close = walkable, can pop back for a kip, chaotic; further away = driving, harder to pop back, more peaceful. Remember, it's a long event & you will need some sleep.
There's always a lot going on at night / early morning, e.g. a semi-big band. Almost everyone who stays in the circuit during the night gets wasted. Most of it is good humoured - walk away from anything that's not - the French and the English don't always see eye-to-eye.
It goes without saying that the traffic on the way in and out is hectic - so make allowances in your travel plans. In the past, the French police were lax about driving antics, but they've started to clamp down. Speed traps in all approach roads, frequent 'pull-overs' to check for drunk drivers. On the latter point, if you need to drive to/from a campsite, make sure the driver doesn't drink. The French can be brutal - instant jail and vehicle confiscated.
The first time there can be a bit daunting, but it's well worth it.