First some definitions. I'm assuming you are American, so I'll use my American view of Paris transportation.
The Metro is similar to the rapid transit systems in New York and Chicago. Sometimes it travels underground and sometimes it travels overground. You must validate your pass or ticket at the entry gate/turnstile. If you are using a physical ticket, do not discard it until you leave the station.
The RER is similar to our commuter trains from the suburbs to the city. In Paris, you can also take the RER between RER stations. You must validate your pass or ticket at the entry gate/turnstile and, also, at the exit gate/turnstile.
The RER trains are all unreserved.
You can be asked to present your validated pass/ticket to an Inspector anytime during your journey on the Metro or RER. Failure to present a validated pass/ticket will result in a hefty fine.
I hope I haven't frightened you from using the Metro or RER. It really is easy. Larger stations will have Transit agents to assist.
- Always validate and keep your ticket until exiting
- If using paper tickets, keep your current validated ticket separate from any unused tickets
- Open gates at Metro stations are either for exiting passengers or the gate is broken
- Don't walk through an open gate without validating your pass/ticket unless instructed by a Transit agent
- Watch what others do when validating to get a better understanding
Saint Michel-Notre Dame is an RER station. There is also a Metro station called Saint Michel. The 2 different stations are connected underground through hallways.
Because you mention Cluny-La Sorbonne, I assume you're hotel is nearby. So you would take the RER B from CDG to the Saint Michel-Notre Dame RER station.
There are various apps you can use to get transit information for Paris including Google Maps. I like to use the Citymapper app. When I'm in Paris I tell the app where I want to go and when. It will give be different options and timing for walking, buses, Metro, RER. Select your route and it will give details including what entrance and exit to use, which section of train to board to facilitate your trip. It even alerts you that your stop is coming up. There are several other options, also.
Your trip to Versailles is included in the Navigo Week Pass valid Monday to Sunday.
Assuming you are both traveling with Smart phones you don't need to be concerned about the Découverte Pass. It is a physical card that you add daily, weekly, monthly passes to. You can load your pass directly to your phone.
Will you be traveling with fairly recent iPhone or Android? It works a bit differently for each.