Are your monitors powered or passive?
If they are passive, you need to connect the Mac to an amplifier, then the amp drives the speakers. You will need a stereo mini-jack to 2 x RCA cable
If the monitors are powered, then you will need a stereo mini-jack to 2 of whatever type of connectors the monitor uses - 1/4", RCA or XLR, depending on the monitor.
Alternatively, if your amplifier or powered monitor set have an optical digital input (mihgt, on higher end equipment), then you can use an optical cable that comes with the Mini-jack Optical adaptor at the MacBook Pro end. Optical output from the MacBook Pro has the potential of being cleaner.
Your next step is to go beyond using the headphone mini-jack output, and use an external USB or firewire audio interface. Search in the Digital Audio forum for many threads discussing these.