How to work the nikkor 50mm 1.8 ais with my nikon d3100?

Pictures are just black when taken

Mount on the camera, set camera to M and start experimenting to get the exposure right. Black means it's too dark so, raise the iso, open the aperture (ring on the lens) or lower the shutter speed.

Your D3100 should also assit you with focussing, look up the range finding function in the manual

You have the wrong lens.

You need to have a Nikkor AF-S 50 mm f/1.8.

Your camera can use legacy AF lenses and still use the light meter while manually focusing the lens, but AI and AIs lenses do not allow you to meter using your D3100. Only D7000 and better cameras can use AIs lenses and meter as well.

There's a table on this link that will help you buy lenses for your camera in the future.

http://bythom.com/lensacronyms.htm