Regarding the RER, some things to note:

1) Unlike the Metro, RER has a schedule. Check the TV monitors for times the trains are scheduled to leave the station.

2) Check the monitors carefully, because they tell you at what stations the trains stop. And note the final destination of each train; they may vary on a line. It's easy to get on the wrong train.

3) The fare varies depending on where you go. There are several zones in the RER system. Make sure you pay the correct fare for your destination. Within Paris it will be the same price as the Metro.

4) Hold on to your ticket. You must use your ticket to get out of an RER station (unlike the Metro).

I've found using the RER within Paris of limited help; it's really designed to bring commuters into the city and out again. The Metro is easier to use and only marginally slower within the city limits.

Bons voyages! Evan in Paris