Many stations have Metro connections if you are coming from elsewhere in Paris. If that is the case, the price includes Paris connections, so you can buy a ticket anywhere you enter the Metro system.
Just to be clear, you must have a ticket valid all the way to Versailles. A regular Metro ticket is only good for the central Paris area; Versailles is out of that central zone.
Also, the RER is a regular commuter rail line so trains run very frequently. The Versailles Rive Gauche Station is the end of that line (it branches off to several other destinations, that's why you must board the Rive Gauche train).
Once off the train, you turn right out the station, walk a couple blocks, turn left on the main avenue and, poof, there is the chateau.
In the gardens down by the grand canal is a bike rental. Bikes make the most sense in the gardens that extend out from there. Rentals are first come, first serve.