Taking the RER (regional train) from CDG airport to Paris
When you arrive at CDG, tired and dragging a lot of luggage, a taxi really is the easist way to get to your destination in Paris. However, the train is a lot cheaper; you're looking at around 12 euros, versus maybe 45-60 for a taxi.
If you want to take the train, just follow the signs for the "RER" in the airport; that's the regional train. (You want the RER B, but that's the only one here.) You'll probably have to wait in a long line to buy tickets. There are machines that are much faster, but my American credit cards seem to not work in them for some reason. Couldn't hurt to try. I try to always arrive in Paris with some euros on me, to make this whole transaction easier; what if my card doesn't work at all for some reason? It's safest to have a little cash on you. But anyway, these machines only take coins, and who's going to arrive in Paris with 12 euros' worth of coins? Not a lot of people. So be ready to wait in the line.
You'll get your ticket and head toward the RER station in the airport. When you put your ticket through the turnstile, make sure to grab it when it comes out the other end, and keep it handy. You may need it later to exit and/or to make a connection in the Métro.
When you get to the train, there will be multiple tracks. Fortunately, the airport is the end of the line, so all trains go to Paris. However, one tiny problem with this is that there are two stations at the airport, in two different terminals, and you may arrive at either one. So, at the very worst, there's a chance you'll get on the wrong train and it will go deeper into the airport for one stop, pick up more people, and then go the other way toward Paris. Seriously, not a big deal at all, and I've made this mistake a couple times. I believe at the station at the very end of the line there are signs in English that say "all trains go to Paris." I have no way to tell you which station is which, so you'll have to follow signs and take your chances; you're only risking a very small delay.
The train ride to Paris takes about 40 minutes or so. The train approaches Paris from the North and stops at seven places. From North to South, they are: Gare du Nord — Châtelet-Les Halles — Saint-Michel-Notre-Dame — Luxembourg — Port-Royal — Denfert-Rochereau — and Cité Universitaire. You can figure out ahead of time which of these is closest to your destination, and be ready to hop off there. Be prepared; you will probably have to push a button on the door to make the doors open; the doors don't open automatically like in the States.
If your RER stop connects with a Métro station, you can use your RER ticket for a free transfer in the Métro. So keep that ticket handy!
If there's no Métro station attached to the RER station where you arrive in Paris, you're gonna have to walk, or cab it, or find a Métro that's within walking distance. Feel free to email me before your trip if you have any concerns and I'd be happy to give you some advice! Once you're done with all this nonsense, it's all very much downhill. You're in PARIS!
Good luck, safe travels,