I read the France Forum all the time and see many of the same questions come up concerning Paris & thought I would give you some of my opinions.
For first time visitors arriving at Charles de Gaulle, take a cab to your hotel especially if you are traveling with luggage. Maneuvering through the transit system for the first time can be a little stressful especially after a long flight & it is easier to learn the Metro system when you have time & no luggage. Do have the name & address of your hotel written down to show the driver. Be sure to get in the official cab line where you will be directed to your cab. Not difficult.
Where to stay: A lot of people say stay in the 5th arr. or 6th arr. to be "near the sights". Well, "the sights" are scattered all over the city from the Eiffel Tower to the Arch de Triomphe/Av. des Champs Elysees to Montmartre to Notre Dame to Montparnasse. All the neighborhoods in these areas have some nice hotels. People criticize the rue Cler area in the 7th as being "too touristy" but I think the popular 5th (Latin Quarter) has been "too touristy" for some time now (not to say you shouldn't see the area). By the way, what is that "meat" the Greek gyro places there are passing off as lamb now?? I have stayed usually in hotels in the 7th because I like walking around the Champ des Mars both in the early morning and at night when the Eiffel Tower is lit. I am now retired, so am able to rent an apartment for a month a year and the company I use has a nice one bedroom on rue Bellan right off rue Montorgueil (check this street out!) in the 2nd. The 18th (Montmartre) should not be ignored as an area to stay in if you really want to feel like you are living in Paris. When I lived in Paris, my room was just steps away from Mo: La Motte Picquet-Grenelle in the 15th, a major Metro transfer point to anywhere in the city and a great place to stay also.
Where to eat: One of the things I don't do is recommend to friends a particular place to eat because everyone should have the experience of stumbling into a restaurant or cafe where they may be lucky enough to have a meal they will talk about for the rest of their lives. It is fun to read the menus posted outside the restaurant or cafe. Every neighborhood has the potential of just such a memorable meal. I will say for all the flak the 7th arr. takes, rue St. Dominique, between Av. Bosquet & the Champ des Mars, has some really great restaurants both on it & just off it. And yes, I still go back to Le Chartier on the rue Montmartre because when you are young and living in Paris on $311/mo, it was (& still is) cheap and they still write l'addition on the white paper table cloths (the endive salade, please).
Just a couple of other things:
If you are going to see the Eiffel Tower for the first time, take the Metro to Mo: Ecole Militaire or Mo: Trocadéro to get the classic view of the Tower from either end of the Champ des Mars rather than going right up to it.
McDonald's & Starbucks - both have public restrooms & free WiFi. Some McDonald's charge a minimal fee to use the restrooms but hop into one when passing by because you don't know where the next restroom will be.
Learn the Metro system. No matter what sights you want to see, you will need to use it to get to it. It is a long way to walk from the Latin Quarter to the Eiffel Tower or to Montmartre. As others have said, the pass Navigo Decouverte is a great deal for anyone staying 4 days or more. You will need a picture (many major Metro stations have photo booths) and it is good Mon-Sun at a time. But it is rechargeable and you can use it on subsequent trips to Paris. All five zones are covered now. Plus it makes a great souvenir!