Those taxi rides to/from the airport will probably cost between 150 and 250 francs, (pronounced frahnh, and be sure to make it nasal - think Gomer Pyle) so be sure you change at least that much at the airport. In fact, changing at least $100US at the airport is not a bad idea. After you arrive in Paris, the best exchange rates are at in-building banks of major employers such as IBM, or at American Express (near the Opera House at the Auber exit (RER) or Opera exit (Metro). The hotel, while convenient, has about the worst rates around. Plenty of places exist just to make change (they have the word "change" ("change") in their names), in the malls, the CNIT, and near any tourist areas.
The subways are much safer than most big city subways in the US, but beware of pickpockets. You will also run into a number of beggars (including some great actors) and musicians playing for whatever they can get (some of them are quite good). Even an insistent beggar will usually give up if you explain in French that you don't speak French (don't ask why, remember?): ``je ne parle pas français''. Maintaining a confused look sometimes helps.
There are three primary components to the Paris subway system: the Metro, the RER, and the SNCF. The Metro and RER are for Paris and the surrounding areas; the SNCF is the country-wide train system. The Metro map looks intimidating at first glance, but is really straightforward. All lines belong in one of the above classifications. The RER lines are A, B and C (with subdivisions - C5 goes to the Palace of Versailles.) Metro lines have numbers, but are often shown with an M (for Metro) in front of them, such as M7. There is a separate RER map as well.
The different directions a line takes are identified by either terminus points, or major stops. Several lines go to Nation; if you can't figure out which one you need, notice which stop(s) it goes through to get there (I believe the M4 goes through Barbes-Rochechoart). Just look at the map to see where you are, where you want to be, and which major stop is listed on the direction signs.
Each train has maps of its line over the doors inside each car, and the stations have their names in 3000 point type every several feet, so if all else fails, get on the train for a stop or two. If you are going the wrong direction, pick a destination in that direction from your handy map, cross to the other side, grab a train, and "voila!" ("you are a turnip!") you are now going in the correct direction.
The RER lines do not stop at every station. Each line has some form of signs indicating at which stations the next (or current) train will stop. If your stop is not lit up or listed, wait for the next train. Repeat as necessary. If you find you are on the side of the tracks which heads in the wrong direction, there will be a way (via stairs) to the other side. The word "correspondance" (untranslatable, as the post office is not involved) refers to changing to other lines. The word "sortie" ("exit") indicates an exit ("sortie").
You purchase tickets for the subway based upon the zones you wish to traverse. Zone 4 is adequate for most things in Paris, including most most of the places you probably wish to visit. Consult a subway zone map if you have any questions about the zones you want to travel. A zone 4 Carte Orange is the best bet on a weekly or monthly basis. Be aware, though, that these are good through the end of a standard time period, regardless of when purchased. A weekly Carte Orange is good through Sunday, even if purchased on Saturday or Sunday, yet costs the same as if it were purchased on Monday! If you arrive on a Friday, Saturday, or Sunday, it's cheaper to buy a daily ticket each day; otherwise buy a weekly Carte Orange (or monthly if you expect to be there a while).
A weekly Carte Orange was 98 francs in early March of 1992. You will need a small photo to attach to the Carte. I got one at Chuy's in Austin; anywhere with a similar instant "4 for a dollar" photo booth will do. Get it before you go over. It's easier than trying to figure out the French instructions, and you get 4 poses instead of 2 (what a deal).
Most everyone travels second class. First class costs extra, and gets you slightly larger seats and (maybe) extra armrests (how many arms do you have?) First class cars have a large "1" ("1") on the wall inside by each door; avoid these if you have a second class ticket. On the RER, first class cars also have a yellow stripe above the doors on the outside of the cars.
Doors on the trains do not open automatically. You must push or turn the latch to get them to open. When the warning horn sounds, the doors are about to close. Subway trains hate people, and suffer them only because they must. Their doors are not like elevator doors; they don't bounce back if they hit something. In fact, when the train door realizes it has caught something, it crunches as hard and fast as it can. I saw more than one weary traveler feed the hungry subway cars; it is not a pretty sight. On the other hand, it does cut down on fuel bills.
Never show fear to a French driver. If you are in the road, assume the right of way, unless at the last possible moment it is obvious they are not going to yield. Like dogs, most French drivers can smell fear, and will home for the kill. French drivers think nothing of going 140 km/hr in bumper to bumper traffic. If this bothers you, close your eyes from the moment you enter a cab until you get out.