What setting did the photographer use to take night photos like this?

I have nikon d90 with the kit lens
i'm taking photos of my friend at night where will be ambient light.

What settings should I use so there's a good balance between ambient light in the background and the focus on the model? I don't want to use the wrong settings so the background is too dark.

Obviously there was some flash involved (camera right as key light and one in the back for the rimming of her hair, probably gelled with a CTO filter). He then set a shutter speed long enough to allow the ambient light to get in.
Check strobist.blogspot.com to learn more about off camera flash

Looks like a 100mm 2.8 with a flash, set the camera to wide open aperture so that you get the DOF, and the flash needs to be remote and the distance to the subject you may need to do trial and error.

You may not have what it takes with the Nikon d90 and lens kit.