As the others have mentioned, using the RER C is a cheap and efficient way to get to Versailles (and not affected by traffic). The cars may be a bit crowded, but it's an easy trip. As someone else mentioned, look for the word "Vick" above the driver's window, as that will be the correct train. Also as mentioned, be sure you have a return ticket to Versailles and not just a Metro ticket.
The walk from the station to the Chateau will take 10-15 minutes. When you get off the train, just follow the herd as they'll likely be heading for the Chateau as well. As you exit the station, turn right for a block or so and then turn left. You'll see the Chateau in the distance.
One other thing - are you planning to buy a ticket there, or use a Paris Museum Pass for entry into the Chateau?