My camera lens doesn't have the A/M focus switch?

I recently bought a new lens for my Nikon D3100. And the lens doesn't have the A/M Focus switch on it. Is there anyway that I can get it to focus automatically? And if I need to fix something in the settings please tell me how.

Which lens did you buy? That would be some helpful info… But if it doesn't have an A/M switch, it's probably only a manual focus lens.

OK, I'm not a Nikon user but here goes from my small knowledge:

The lens might be made for other Nikon DSLR's that have in-body autofocus. These other DSLR's have the focusing motor in the body instead of the lens doing it.

Your particular camera has no in body focus motor, so it depends on lens autofocus. But your lens is probably made for the other type of camera mentioned above. That means you can only use the lens in manual focus.

The D3100 needs lenses with built-in motors, which this one doesn't have.

If you want autofocus, look for a motorised lens (Nikon removed the body motors from the D3000/D5000 series to make them cheaper).

This problem doesn't exist with Canon (all lenses motorised) Pentax, Sony or Olympus (all have body motors, some lenses with motors), but it's a headache for Nikon users on a budget.

Nikon cameras with built in motor like the D7000 have a switch on the body. If you are using the AF-S lenses you would normally leave the body switch on. You can turn autofocus on or off using the lens switch. If you have the older type AF-D lenses, those lack a motor in the lens, and they lack a switch on the lens. The body switch is used to turn autofocus on or off.
With a camera like the D3100 that has no body motor, there's no body switch. With the AF-S lenses you turn autofocus on or off with the lens switch. If you put on an AF-D lens with no motor there's no autofocus, so there's no need for a body switch to tell it whether to autofocus.