Guide Number (GN for short) = distance to subject times aperture (f/stop) at ISO 100.
or to put it in a more useful equation:
f/stop = GN divided by distance to subject at ISO 100.
so if the GN =120 (in feet), your ISO is 100, and your subject is 10 feet away, then the correct aperture to get normal exposure would be f/12 (just round down to f/11).
If you are using a different ISO, just adjust by as many stops difference from ISO 100. In the above example using ISO 400, you'd go to f/22. If you were using ISO 50 you'd go to f/8.
pay attention to whether your GN is in feet or meters
typically fill flash is 1 stop underexposed compared to background lighting, just adjust f/stop accordingly
shutter speed has no bearing on the exposure of the flashed subject (except your camera's max sync speed), but is used to control background exposure (as long as the background is far enough away that the flash won't really affect it).
