What lens is good for a beginner photographer?

What lens is good for a beginner photographer and that will fit a Nikon D3100?
If you could answer this:
Also, how do you know if a lens will fit your camera?

The kit lens that comes with it is a pretty decent lens for a beginner. It does not have much zoom with it, so you may want to get a zoom to go with the basic 18-55 kit.
Your camera does not have the auto focus motor in it so you will need to purchase lenses that do have it-or it will be manual focus. Not an easy trick to learn.
I don't shoot nikon so I know only a little regarding their lenses, but I *believe* they are marked with a G. Hopefully one of the Nikon guys will be able to help you more.

Definitely the kit lens. Get familiar with that before you start purchasing additional lenses.

It depends on your type of photography. In all cases, if you are serious about photography, don't buy something cheap now planning on replacing it later when you are better… That's just throwing your money away. Invest in yourself.

For Portraits: Get a fixed zoom 50 mm lens with as low of an F stop as you can. You'll be happy with the result.