What aperture do I get when I use the 18-55mm G lens on Nikon EM body?

Smallest.

If the smallest for that focal length is f/22 then that would be it. The G spec means there's no aperture ring so aperture is stuck at smallest.

http://www.kenrockwell.com/nikon/compatibility-lens.htm#cheap

And yes, the lens won't fill the whole film frame so you'll end up with small dark pictures with equally dark, large and fuzzy borders.

F3.5-5.6, depending on the zoom range, and a smaller image in the centre of the negative.

A 35-70mm or 28-80mm in Nikon AI fit would be stupidly cheap on the used market, though, and would do the job a lot better - particularly as the EM is Aperture Priority only.

Note that it's a DX lens, so it wouldn't be able to fill the whole frame of a film camera. There will be serious vignetting around the center of the photo.

Why on earth would you want to do something so goofy? You have no control of the aperture at all with that body and lens combo, and the lens is designed to work with crop sensor digital cameras.

An old Ai or E series lens is dirt cheap, just like the EM body.

The Nikon super electro 5000 plus