Paris' Metro, or rapid transit system, was inaugurated in 1900 and the core completed by the 1920s. I really love the Metro and feel comfortable using it; once you figure it out, it's fairly easy to navigate. In fact, for me, it is easier than the Tube in London and the subway in NYC. But don't let me fool you--I remember how intimidated I was when mom amie, Cindy, gave me lessons on my first trip in 1993. And I still have to pay particular attention when I take the RER (the suburban arm of the Metro). Girl from Iowa never had much of a chance to use public transportation before Paris! Each line or route has a number; is marked by a different color; and is also known by the two ends of the line so you can determine the direction you need to go. For example, line 9 is lime green and is identified by Mairie de Montreuil and Pont de Sevres. Line 8 is lavendar and the end stops are Creteil-Prefecture and Balard. If you're entering the Metro at a spot on line 8 and your destination is towards Creteil-Prefecture, then you must make sure to get on a train going that direction (and not the one towards Balard). There are information maps of all the lines and stops when you enter the Metro so before you submit your ticket into the machine, you can be sure you're at the right place and/or know which line(s) to take to get to your destination. Once you're on the train, on both sides of the train above the doors and/or windows is the line, all its stops, plus the lines that branch off, if any, from the appropriate stops. Because obviously, you may have to take line 8 to Republique, for example, and change to line 5 to get to your final destination (called a Correspondence, which is well marked, as well). If you notice after your train ride begins that the first stop is really one going the opposite way you want to go, just get off, cut back around in the station and get on the train going in the correct direction (no additional charge).
When I first started using the Metro and the end stops were hard for me to pronounce or recognize, I would use word association to remember which end of the direction I wanted to go. For example, if I wanted to take line 8 towards Balard, I would think "Ballard" as in Seattle. La Courneuve 8 Mai 1945 became "May 8th;" Boulogne Pont de St-Cloud became "bologne." You get the picture. I enjoy the newer trains that actually announce each stop as it pulls in--allows me to practice "hearing" the pronounciation so that someday, I might be able to understand the French! Some of the stations are pretty run-down and some are fancier, like the ones near the Louvre with their faux works of art. 
If the doors don't automatically open when you get to your stop, either press the green button or pull up on the lever on the door. Upon disembarking, you are free to toss your ticket--although I've never seen anyone check to see if you have a ticket, legally they can. Look for the closest Sortie (exit) because there may be several and you can save lots of walking by choosing the best one. Fortunately, there are plan du quartier maps posted on the walls so you can see where you want to emerge above ground. Stations often have many stairs--today I counted 60+ as I climbed out--and oftentimes the escalators, if any, do not work. I only notice this when I'm carrying luggage... How do you know which line(s) to take and the station to get to where you want to go? Addresses, advertisements and listings typically include the closest "Mo" stop, which is very helpful.
Tickets--a single ticket good on the Metro, RER, and buses is still 1,40 euros (I think). You can save some money by buying a carnet, which is 10 tickets for around 11 euros. Automated machines and attendants (most stations) in the Metro stations sell the tickets. This time I bought a Carte Orange mensuelle, which cost 52,50 euros and is a monthly pass good for unlimited travel (my sister tested this last year using a weekly one--starting Monday and ending Sunday). So if I ride at least twice a day--and I do more than that usually--well, you do the math. I brought a passport-sized picture with me--required for a Carte Orange--but some of the stations have photo booths (I need botox...). On February 1st, I'll buy another single ticket for another 52,50 euros. And then in March, since I'll be leaving on the 8th, I'll buy just a carnet because I also had some single tickets left over from my trip in September. Based on my research, the Paris Visite passes designed for tourists are overpriced.
Viola! In a nutshell, that's a basic lesson on the Metro. Oh I wish I could live some place where I never needed to own a car! (PS - I have not taken any buses yet but will do so on this trip. The Carte Orange works the same for each bus ride.)
