There's a great post for first timers on another forum - the UKCS website - Advice for continental first-timers. Should tell you everything you need to know.
You need ownership documents for car, caravan and anything else of value.
Driving Licence, MOT certificate, etc. If it's a company car you need written permission to 'export' it.
Health Insurance.
Breakdown Insurance - make sure it covers van too.
Passports, obviously.
Continental adaptor plus polarity tester and 'reverse polarity adaptor'
and probably a lot more. Trekkin Tekkie's post gives loads of great information.
Dover/Calais ferries are cheapest.
Try viamichelin to work out a route - don't forget to select 'with caravan' on the 'options'.
If you can go outside last two weeks in July/first two in August then things become less hectic and you can find a better choice of pitches.
Depending on your route, and where you're heading you'll need at least one and possibly two overnight stops. Divide the distance into thirds and plan stops at these points.
Plan regular comfort stops - and take no notice of the time the website tells you the journey will take - these websites calculate the whole journey at the maximum speed allowed, and allow no time for stops at all.