Read the manual that came with your monitors. The manufacturer lists several procedures, warnings and precautions that should be followed for the proper operation of the monitors. This increases monitor longevity.
The distance of the monitors from nearby walls will determine how loud to set your levels. If they are too close, the sound will reflect off the wall. The suggested minimum distance is 20 inches. If you have powered monitors, then this allows enough ventilation for the heat sink located on the back.
When studio monitors are placed on a flat, hard surface, some of the sound travels through the surface, diffusing it. This can lead to turning up the levels louder than what's needed. Investing in a set of isolation pads and placing them under your monitors will result in a more accurate sound.
Monitors should be placed at ear level, pointed toward the engineer's chair, equidistant from one another. When the engineer's chair is turned toward them and is sitting in a certain place (commonly known as the "sweet spot"), the full spectrum of sound can be heard. Improper placement will lead to mixes that are too loud, muddy, or a whole host of other sound problems.
Set all the faders on your monitoring software that came with your sound card to 0 except the master faders, which should be turned all the way down. Play some music in your DAW and slowly turn up the master fader. Look at the input monitoring meters. A steady hit in the yellow is fine. Avoid turning things up so loud that they go into the red. This, called clipping, means it's too loud.