Advice for first timers in France please!

My partner and I are intending to take our caravan to France next June/July and are currently trying to get our heads around what to book first - the ferry or the site? How early do you really need to book the ferry and is it best to get this done first before choosing/booking a site? Also, any recommendations for a nice site in Brittany/Atlantic Coast?

Lots of questions I know but thanks in advance. Reg