My pictures turn out black?
I have a Nikon D5100 and I'm just learning how to use it. The lens cap is NOT on. I was trying to figure out how to adjust the ISO, aperture, and whatnot; but now the pictures all come out black when I'm in the manual mode. It takes pictures just fine in the other modes, but I can't adjust all these things in other modes. Is there something that I need to adjust in the manual mode to make the pictures turn out normally?
In manual mode you have to balance everything yourself.
When you're using full auto, then the camera works out both the right aperture (size of the hole in the lens) and shutter speed. When you use aperture priority, you decide the aperture and the camera works out the shutter speed. Shutter priority is the other way round. In all these cases, the camera is helping you out to a greater or lesser degree.
In manual, the camera is not helping you as much. You have to change some kind of combination of ISO, aperture and shutter speed until the camera's meter indicates that your exposure is right. The meter is a gauge with a zero in the middle, and plus and minus signs on the left and right. The nearer to the middle, the closer you are to a good exposure. If the meter is showing all the way to one way or the other, then you have a picture which is massively overexposed (all white) or massively underexposed (all black, like you are experiencing). Sounds like the numbers you're dialing in are leading to heavy underexposure, i.e. Not enough light.
Learning to shoot in manual mode also involves learning to train your eye to get a rough idea of the right settings for the scene. If it is a bright day, for instance, and you're at ISO 100 with your aperture at f8, you'll gradually get an idea of an approximate shutter speed. Same if it's dark and you're at f2 at ISO 800, etc. If you have no idea at first, you'll have to experiment with the settings until the meter indicates that you're in the right area. With time and practice you will get a sense of these things.
This article will tell you more about metering.
http://annelosch.hubpages.com/hub/The-Exposure-Meter
Oh dear. I guess you haven't read the user manual yet, the page that tells you how to meter the shot?
You are probably using a shutter speed that is to fast