Stage lighting can be tricky, at times.Many times there are spot-lighted actors against fairly dark backdrops; those conditions often cause severe pver-exposure of the actors. Metering can be tricky for the beginning shooter. With shorter focal length lenses 50mm and shorter, at times you need to set the meter to a Minus 1.7 EV setting, to correct for the spot-lighting. White balance can also be tricky, and usually the lights are not daylight balanced, but more in the 3400 to 2900 degrees Kelvin range. If there are actors in front of a dark, un-lighted background area, the loght meter could be over-exposing them by as much as 3 or 4 or even 5 EV, if they are spot-lighted, and there is a lot of the dark area that the camera metering field "sees".
Spot metering can help get the right baseline meter value for a scene. Something helpful is to shoot the dress rehearsal, and also to know ahead of time how the play will be lighted; just like the actors, the lighting crew has its own, pre-established routine. Some scenes will likely be lighted one way, others in a different way.
Sometimes a play is not really low lighting, but rather not a lot of light all over, but it has the spot lighting on certain places, or flood lights that leave large dark areas, but the actors will be in pools of light. Shoot for the actors, shoot mostly for the highlights. Shoot in RAW mode if at all possible. Do not be afraid to go to ISO 800 to 3200 or ISO 4,000,5,000, or even 6,400 is your lens is slow in aperture.
Think about the rough baseline exposure for the various scenes,like say ISO 3,200 at f/4 at 1/100 second. Many scenes will be in some kind of a general,overall lighting level, and if you know it, you can use a manual exposure and set the scene exposure and shoot without fiddling with exposure. Keep in mind: you need to keep the shutter speed high enough to stop movement and prevent camera shake/blurring, so try to keep things at 1/100 second if possible, or faster.
If the play really is one of those dim-dim-dim ones, you might have a heck of a time. Consider the dress rehearsal to be a MUST-shoot opportunity for you if you've not shot a play before. At that, you might be able to set up a tripod, stage center, 8 to 10 rows back, and be able to snipe off shots that are good, which is very different from shooting during an actual performance.