Beauvais is a city in France.
There are two options, the first is the D1001 which is right beside the airport and probably the best if you've just flown in (see below). The other one (also for Paris) is to walk to the péage at exit number 15 on the A16. There's not a huge amount of space for cars to stop, but it works. Try and stand on the tarmac island right where cars can turn around and go the opposite direction.
To Paris from Beauvais Airport
When you get out of the airport turn left and walk for 5 minutes or so Route de l'Aéroport street until you get to a roundabout. At this roundabout turn left and you will see a dirt area to the right side of the road D1001. Here it should be dead easy to get lifts, and will save you the ridiculous 15 euro ticket into the city. It's probably best to turn down lifts going into the town of Beauvais, because then it can be difficult to get back to the highway. Just be patient and eventually you will get a car going directly to Paris.