Always shoot in manual mode when using external flash. This will solve your problem of on board flash from popping up when external flash is fixed.
Keep your ISO to the minimum. Set it to 100 or 200 max.
Vary your shutter speed and Aperture to get the right exposure.
A tiny hint for beginner in flash Photography: Shutter speed controls the brightness of ambient light in your image. Increasing the shutter speed decreases the ambient light in the image; decreasing the shutter speed increases ambient light. I would suggest you to keep your shutter speed faster then 1/125 seconds at least.
Aperture controls the brightness of the flash.
Keep this in mind and try. :) Happy clicking mate.
