How do make my images lighter?

Basically, I recently bought a Nikon D3100 and I'm quite new to the whole photography thing. My problem, however, is that whenever I point the camera at something that is even the tiniest bit dark, it will look fine on the screen, however, when I take the picture and look back at it, it will be almost completely dark! I've tried changing all of the settings and everything but none of that helps. When there's plenty of light, the picture looks the same both on the screen and when I look back at it after I've taken the picture, the problem only occurs when there's just a hint of darkness.

The camera takes an average reading of what you see in the viewfinder and makes an exposure based on that… For most 'shots' average readings are fine… If your 'subject' has bright light behind it then the camera meter will be 'fooled' and you have to learn how to compensate for this…

If you move in towards the subject when the lighting is tricky and fill the frame with the most important part of the 'scene'… If it's a portrait take a reading from the persons face… With the information displayed in the cameras viefinder you can set the camera to expose correctly - some cameras have a AEL (automatic exposure lock) that when you press it… It holds that reading in the camera memory until you press the shutter… This gives you time to recompose or reframe your shot and will give you a much better exposure…

If you are struggling with my 'explanation'… Borrow a photography guide book from your local library and have a read at it… The instruction manual used to cover 'tricky lighting' and explain how to adjust the camera for better results… Exposure compensation and bracketing can help get better results too.

Camera take average reading if its set to average, you could have it set to spot thefore it will take value from only where you point camera.

my d3100 was pretty much spot on with exposure.

it could be you have a wrong setting somewhere so would suggest you take a short photography youtube video course.

you can use exposure compensation to adjust brightness
but yes sometimes on screen preview and aftershot can be different.

most dslr users use the viewfinder and then look at screen to sample photo taken if its too bright or to dark then exposure compensation can be adjusted.

in fact this is photography, take photo, check screen, adjust
exposure compensation to suit, retake photo.