There isn't really a magic formula like rio said, but I would definitely say to keep the iso low. I prefer iso of around 200 myself, but unless you have a camera designed for low light, higher iso settings will add a lot of grain.
Shutter speed and aperture are the two I tend to change before I touch my iso. I'd invest in a tripod if you haven't already, which will allow you to play along with your shutter speed and have longer speeds. I mostly use my shutter speed to control the light the comes in the camera. The faster, the less light. I tend to keep my aperture at a lower f number, usually f1.8-7.1 to keep a shallow focus, however if you need to close your aperture up and still want shallow focus, you can achieve this with a longer lens, and zoom in on the subject from a greater distance.
You can use more light also, but don't let it change the feel of your scene. If you're happy with what the lighting looks like, play around with the camera to capture the light before you change it to work with the camera.
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