1) Credential:
Usually, pilgrims bring one from their country - or, obtain one at the first albergue (pilgrim shelter) they stay in when starting.
As you don't know where you will be starting - only somewhere in France - you could email one of the French Associations and ask them where to get a credential. I say 'one of the French Associations' because who you ask will depend on where you start . (Nothing is ever easy!!)
Once you know where you are starting, email one of these associations:
France: Société des Amis de Saint Jacques de Compostelle:
http://www.compostelle.asso.fr/
France – Vezelay: "Via Lemovicensis" (La Voie de Vézelay):
http://www.amis-saint-jacques-de-compostelle.asso.fr/
Southern France: Association de Coopération Interrégionale les Chemins de Saint-Jacques-de-Compostelle
http://www.chemins-compostelle.com/
France – Chartres: Association des Amis de Saint Jacques d'Eure et Loire
http://monsite.wanadoo.fr/chartres_stja ... ndex.jhtml
France - Saint Jean: Au coeur du chemin - les Amis de Chemin de St Jacques des Pyrénées Atlantiques
http://www.aucoeurduchemin.org/
OR: you could email Mundicamino for advice:
mundicamino@mundicamino.com
2) Where to start in France?
You don't say how long you have to walk to Santiago so it's difficult to advise you on where to start. There are at least 5 routes from France that lead to Spain and then to Santiago. You'll need to decide which one you want to walk.
a) You could start in Paris (a very quiet route) and walk the Via Turonensis +970 to Spain and then the
Camino Frances 800km or the Camino Norte 765km to Santiago.
b) Or on the Via Podiensis from Le Puy to St Jean - 736km and then 800km to Santiago
c) Or on the Via Tolosana from Arles - 745km in France and about 170km to Puente La Reina and then 685km to Santiago
d) or the Via Lemovensis 900km from Vezelay and then 800km from St Jean to Santiago.
e) Or the Chemin du Piemont ± 525km from Narbonne via Lourdes to St Jean and then 800km from St Jean to Santiago.
There are a few other shorter routes that cross the Pyrenees from France into Spain and join up with a longer Spanish routes but no need to confuse you with those!
I hope this helps a little to answer your questions.