My camera won't work properly with the 50mm 1.8 Mikkor?

I've had the Nikon D3100 since Christmas, and I finally bought a new lens for it. After seeing many good ratings for the 50mm 1.8 Nikkor, I decided to get it. Once I got it, I tried testing it out. I can take pictures with it, but only on the minimum aperture. I tried putting it on different apertures, but My camera told me that It needed to be on the minimum aperture. Is there anyway I can make it go on different apertures?

Let me guess - you got the 50mm/1.8D AF lens, not the 50mm/1.8G AF-S, right?
If it's the former, then you should return it and get the AF-S version instead. That's because the AF version won't autofocus on your camera, while the AF-S version will.

Anyway, the AF version that you likely have has an aperture ring on the lens. That one needs to be set to f/22 to use the lens on modern cameras. This however does not mean that you can't control the aperture, you just don't control it on the lens using that aperture ring. Instead you control it from the camera, using manual mode or aperture priority mode on the big dial on top (M and A respectively). Then you can control the aperture using the thumb dial. Note that in order to change the aperture in M mode you also need to hold the exposure compensation button, as otherwise the thumb dial changes the shutter speed instead.

Had you read your user manual, you would have noticed that you have to provide the camera with aperture and shutter speed settings that produce correct exposures. You will have to use the light meter in the camera to do this.

It is obvious that when you open the lens aperture all the way, that you are being told that your shutter speed is either to fast or too slow.

You really need to read the part of your manual that descusses how to pick the right exposure.

If you shoot in Manual mode you can set the aperture to anything you want.

If the lens has an adjustable aperture ring, that means your lens is the AF-D version, and has no focus motor.

You won't be able to autofocus with that lens since your camera doesn't have an internal focusing motor. With your camera, you would need the more expensive AF-S 50mm lens, which does not have an aperture ring.

However, you can still control the aperture. For any Nikon AF-D lens, to allow the camera to control the aperture, you must always put the aperture into the minimum setting. When that happens, the camera will control the aperture - even though the aperture ring is set to minimum. Setting the aperture to minimum is simply a mechanical function of the lens that puts it in the mode to be controlled by the camera.

In fact, you will see a little lock on the lens so that you can lock the aperture at the minimum setting so that you can't move it by accident. But the camera will control the aperture regardless if the lock is in the up or down position, as long as the aperture is set to minimum.

If you move the aperture ring to another setting, you are telling the camera that you want to override it's control and you want to set it yourself. At that point, the camera will blink "F--" unless you put it into manual mode - or move the aperture back to it's minimum setting.

Think of it in this way. The aperture ring on the lens will set the aperture, unless you put it into the minimum setting. When you do this, you are telling the camera to control the aperture.

Make sense?