Using the Metro is very easy and will help when you want to get around, especially getting over to Sacre Couer.  You can buy tickets individually or in a batch of ten (un carnet).  They get stamped as you enter the Metro.  You use one ticket per journey within the main area of Paris (you will need to take a train to go to Versailles or other outside-the-city areas).  The ticket is good no matter how many changes you make, but once you leave the metro it expires.  Hold onto your ticket!  I recently read that if you go to a ticket machine to purchase, you will need one of the new credit cards that are chip-and-pen and NOT the chip-and-signature version.  I called several banks and many U.S. financial institutions do not have the chip-and- pen card yet.

Also, when it is cold outside it can be a welcomed mode of transportation over walking.  We were in Paris in January a few years ago, and found the Metro to be packed with travelers who also wanted to avoid a long cold walk.  Please be prepared for this in the cold months, or during heavy rains.