Can anyone advise what rain I should use to travel from CDG to the St. Germain district of Paris
http://www.ratp.com/ is the website where you can look up iternaries for Paris...that should help! There is an English version, just click on the British flag.
The RER B line is the only one serving CDG. Take it to St-Michel Notre-Dame. What Metro or RER line to take from there depends upon exactly where you are going in Saint-Germain-des-Pres.