I know it might sound like an easy thing but I often get asked by my English clients when they move to France how to write a French address. It is only easy if you know the answer! Here is the answer:
Start by putting on the same line the title of the addressee: ie M., Mme or Melle, then the surname in capital letters then the first name if you know it.
On the 2nd line, the company name if you are sending a business letter.
On the 3rd line, the street/road number and name.
And on the last line, the post code and town or village name in capital letters.
I often see English people adding the area where they live ie Brittany, but there is no need for it.
Here is an example of a French address:
Monsieur DUPONT Pierre
5 rue de la mairie