You can catch the metro/train from various places throughout the city.
If you just have the normal Zone 1 metro pass then you will need to buy a single or return Zone 4 metro pass to get to Versailles and back. This should cost something like €5.30
When we did it we went to Champ de Mars/Tour Eiffel and bought the tickets there.
There are other more cost effective options if you want to visit other places.
http://www.ratp.info/orienter/cv/carteparis.php - interactive Paris metro map
http://www.ratp.info/touristes/index.php?langue=en&rub=decouvrir - all the different metro ticket types you can buy about 10 years ago
We did a 3 hr tour and paid 50 Euros per head. Do not recommend. You will need more time. The day long tour is ideal. The bus gets there in 45 minutes and it is no hassels. The entry and the audio guide (think it was free) is included. The bus office is near the Joan of Arc Statue in Place des Pyramides. about 10 years ago