Always shoot in manual mode when using external flash. thisThis will solve your problem of on board flash from popping up when external flash is fixed.
Keep your ISO to the minimum. setSet 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 your over allthe brightness of ambient light in your image . fasterIncreasing the shutter speed darker your image. slowerdecreases the ambient light in the image; decreasing the shutter speed brighter it getsincreases ambient light. iI would suggest you to keep your shutter speed faster then 1/125 seconds at least.
Aperture controls the intensitybrightness of yourthe flash.
keepKeep this in mind and try. :) happyHappy clicking mate.