Used to live in Europe for several years and driven on both sides of the road with left and right hand drive cars.  2 main things.  Never - never stay in the fast lane ( or left lane in France) unless you are passing.  If you do that you will get flashed multiple times and no doubt some nasty words being said in some language.  Europeans strictly respect the law that you only use the fast lane for passing.  Annoys the hell out of me to see people hogging the fast lane in the US.  Same goes in busy city freeways.  AND get used to roundabouts quickly.   Don't stop before entering a roundabout unless there is a car in the outside lane.  You might get rear-ended.  Drivers are fast and they will be watching the roundabout in front of you and not expecting you to stop for no reason.  Not sure who would be at fault but you would rather not find out in the first place.  Driving in Europe is serious business and not a time to chit-chat, look at farmers fields, drink coffee and all that.  My wife is European and it drives her crazy how Americans drive.  Had to buy her a Subaru so she could have a manual.  She drives that thing like she is in a rally race.  But she is a lot safer than most American drivers.  Of course when you are on as small country road with little traffic enjoy the scenery.

In rural area farmers do not have trucks.  They use tractors to haul everything and just to drive from home to farm.  They have license plates and are inspected just like cars.  So it is not uncommon to get in a line behind a tractor when driving in rural farm country.  Hopefully you are not behind one pulling a manure spreader.

Driving through downtown Paris is definitely a challenge.  The Champs Ellyess is a must once you get the hang of driving.  Some of the roundabouts are 5 or 6 lanes.

As others have said, get an international drivers license.  Also check what your insurance situation its.  I use American Express to hire a car because of their insurance.  Had someone hit my car in a parking lot in Scotland.  Called American Express and they took care of the whole thing.  Never heard a thing about it afterwards.

Most of all, enjoy.  By the time you leave driving in Europe will seem normal.  Just a bit nerve wracking the first couple of days.