Debbie,
This question seems to occur on a regular basis. The website provided in the previous reply will give you a good idea on the options for the trip into Paris.
If you're planning to use the RER / Metro method, you can purchase tickets either at a staffed ticket office (note that there are two at the airport, however one is ONLY for TGV tickets etc, and the other for RER tickets). Depending on the number of flights arriving at a particular time, the staffed ticket office sometimes has a HUGE queue! Tickets can also be purchased via automated ticket Kiosks (usually the faster method). However, these will ONLY accept the newer "chip & PIN" credit cards or (as I recall, Euro cash). I used a "chip & PIN" credit card when I was there last June and it worked well.
Happy travels!