You have quite a few options when it comes to buying transit and museum passes. The RATP has a site with lots of information available in English:
http://www.ratp.info/informer/anglais/index.php All the cards below can in theory be bought at any Metro station, but the chance of them actually having it in stock and maybe being able to help you in English are better at larger rail stations or FNAC. This goes without saying, but never buy a ticket from someone who "just happens to have an extra," no matter how trustworthy they may seem. As much as it saddens me to say this, you should probably not accept help operating the ticket machines either, as this can be a ruse of pickpockets.
The museum pass (
carte musee' is separate from transit and is bought in 1, 3, and 5 day increments. The days are consecutive, so you have to use them all in a row. The main benefit is that you don't wait in lines. This is worth more than its weight in gold. Maybe not in the off-season, but during the summer the lines for the biggies like the
Louvre and
Musee d'Orsay can be hours. With the
Carte Musee' you can just walk in (after going through security). Expensive (EU18/$23 for 1 day, cheaper per day for 3 and 5 days) but worth it when you consider what you're paying to be awake in Paris for an hour.
As for transit, they will try to sell you the
Paris Visite. This is essentially a rip-off aimed at tourists, unless you make heavy use of the metro outside the center of the city (not likely) and take advantage of the museum discounts (if you're going to that many museums, get a museum card). It costs EU26.60 / $35.40 for five days. The
Paris visite is a good deal if you are going for fewer than five days with children (ages 4-11 are EU13.70 / $18.20 for five days).
The
Carte orange is almost always the best deal, if you can take advantage. It costs EU15.40/$20.50, runs from Monday to Sunday (and can't be bought after Wednesday) and allows unlimited travel in the downtown area, whether on metro or bus. Ask for the "carte orange hebdomadaire" for zones 1-2. You will have to buy a separate ticket to go to the airport or
Disneyland Paris. Still, with the aid of a pocket map you can get completely across town in 20 minutes or so. The only caveat is in theory you need a passport photo to affix to your card. I've never bothered, and I don't think the
controleurs who occasionally inspect tickets will hassle an obvious tourist about that.
Or, if simplicity is what you crave, a
Mobilis card is a plain 1-day pass for EU5.30 / $7.
I mentioned a map, the slim, inexpensive
Plan de paris will show you Metro stops and street maps, and fits in a pocket, and can be had for a few euro at any bookshop or place that sells maps. Actually, my sister-in-law found one at Restoration Hardware, with a nice vinyl cover and everything! Not in English, but you don't need to know French to use the handy list of museums and the closest Metro stops.
One tip: Don't overlook the bus. It's harder to dechipher the routes, you can get stuck in traffic, and like most places, the crowd on the Metro is somewhat less likely to be... unsavory. However, you get to see more of the city itself.
Have fun!
Walt
P.S. Oh, no, I see you already left. Hopefully you were able to get your rail pass successfully.