I concur with catchlights about the 2 stops. Here my suggestions, please do your own tests before you start shooting. For all settings check your manual if you are unsure. It is possible that certain settings are not available on 100D which I have on 50D.
Mode: Av, -2EV, ISO 800 (as start), Center weighted metering (or spot) => make sure your subject is in the center
Settings: f = 5.6 (otherwise DoF too thin), 0FEV
AF: center focus point
Check that you get shutter speeds above 1/60 to freeze motion. Actually, the flash freezes motion nicely ('flash speed' is much faster than shutter speed). The shutter speed determines how much ambient light comes in. If it's too slow, you get 'light streaks' (which can be a nice thing, too). If necessary, increase ISO or open aperture. Using Av will keep the camera in charge of shutter speed, depending on ambient light. This will result in a consistent ambient exposure even though the lights might change when you move around.
If the ambient light is evenly spread you can also use the readings from Av and feed M mode. Then you need to keep an eye on the exposure meter (below viewfinder) to avoid overexposure by ambient light. But in return you have more control over the balancing of ambient and flash light: fast shutter for less light, slow shutter for more light. With second curtain flash and slow shutter you can get nice streaks, creating images of motion (also called 'dragged shutter').