For first-timers I'd suggest you get a good map (Michelin Atlas to France is a good one), and get (or borrow) as many guides as you can. The Alan Rogers guide is a good one, as it the Caravan Club guide to Europe part 1. The Caravan Club guide is great for first timers because it gives lots of advice sections, from what you need to take for the car, to a packing list, tips on electricity, and many others - definitely worth the money. Camping Cheque give you their book but the directions to the sites can be hard to follow (we don't have GPS so we can't use their references). Sometimes it's best not to decide, but go where the weather, the mood, and other campers' recommendations take you. Start with a site not far from your port of arrival, then you have plenty of time to set up, and get a feel for the place. If you like it stay a couple of nights, and talk to others - find out where they recommend. Fellow campers often have the 'best sites' to recommend. Most of the Camping Cheque sites we've stayed on have been excellent - some better than others, of course, but everyone likes their own particular things about a site. If you're going to Calais then I can recommend Chateau de Ganspette - if you go there ask for a pitch in the newer section - bit pitches, excellent toilet block, and a good meal in the restaurant. They all speak excellent English and there are lots of other overnighters as well as regulars who stay longer.
Forge de St Marie at Joinville is another Camping Cheque site, handy for the route south if you're going to the east. We don't do the west coast much, because we're not beach people, and if we do swim in the sea prefer the warmth of the med. Don't take too much with you - but food, wine, etc., locally. Take half as many clothes as you think you'll need.
Three weeks should be long enough to get around to any area of France you fancy. There is something to suit everyone - we love the mountains of the Massif Central, the Cevennes and the Ardeche, as well as the upper parts of Provence - and there are some lovely sites (Camping Cheque offer many) in these areas.