How to adjust brightness.nikon d3100?

How i can adjust brightness when taking picture in my nikon d3100 camera.

For your screen, it'll be in the menu.

For exposure, that's what the aperture's for.

Either will be in the manual, if you don't have a manual, it's a free download from Nikon.

It just amazes me how many people buy a DSLR and haven't a clue on what shutter speeds/apertures are for. In addition to reading your manual, I suggest buy or borrow a book from library on Basics of Photography.

If you want the image 'brighter', ie slightly overexposed, then in Auto modes (including A & S modes) you'd use the exposure compensation button (+/- EV) and set it to +EV.

In manual mode, look at the light meter & adjust the parameters until the meter is reading on the + side.

Read you camera manual & take a basic course in photography if you're going to use a DSLR - otherwise, sell it & buy a point & shoot.

Apparently, reading & studying the Owner's Manual for your camera hasn't helped. Perhaps this video will: http://www.nikondigitutor.com/eng/d3100/index.html Who better than Nikon to teach you to use your Nikon D3100?

Also, learning about the Exposure Triangle will also help you improve your photography.
http://www.digital-photography-school.com/learning-exposure-in-digital-photography

For what its worth I shoot in Aperture Priority about 95% of the time.