Studio monitors are usually high-quality speakers, so treat them to a decent amplifier. Then, as Chris has outlined, run a stereo audio cable - but to the amp - and hook the speakers to the amp as well.
Example: I have two Technics SB-CD911 speakers running from an Aiwa KSX-V70 amplifier, connected to my eMac with a plain stereo 3.5 mm to RCA plugs. The sound from iTunes is very good, even better from VLC. Best quality comes from audio file format such as .AIFC or .m4a, or .WAV.