I think you need to tell us how much light you had to work with or was it pitch-black dark?? I would first suggest to get your lens as wide open as possible; therefore, perhaps the best thing to try, is A mode. Your shutter speed will be dictated by the camera. You can go auto ISO or try somewhere between 400-800. Put wite balance in auto. Try a few shots and see what works. From my experience, the photos will be a bit "soft" because you are not using flash. The 5100 is a master in low light. You just need to play around with your aperture and your shutter speed in order to let in the right amount of usable light.