France is a great country for hiking. There are many friendly car and truck drivers. The highways cost money, and at some péages (toll points) you can get a hike easily.
At some Péages all the traffic has to stop and pay - these are excellent spots to get a long ride further. Most cities next to a highway have a péage where all trafic has to stop.
You can get free maps in the péage offices - these also indicate where you can find the "all-stop-péage".
Some Péage are really good, some not so good. If you've been waiting for a while with an indication of where to go, drop it and try with your thumb only. And also, you can try to get a ride to the next good spot in the wrong direction.
Most of the French don't speak English. So have a map to show them, where you want to go. If you know any French word, use it! French people like it, if you try your best. Say "Bonjour Monsieur/Madame", to show that you are friendly. If you're heading to Germany, you maybe need to know that Germany is "Allemagne" in French.
mappy is a good online map for France, it shows you (to) where you can take public transport.
French number plates end with the number of the département the car is registered in. For example, Parisian cars end with the number 75. See List of arrondissements of France and French vehicle registration plates at Wikipedia.