1) 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.

2) Keep your ISO to the minimum. set it to 100 or 200 max.

3) Vary your shutter speed and Aperture to get the right exposure. 

A tiny hint for beginner in flash Photography. Shutter speed controls your over all brightness of your image . faster the shutter speed darker your image. slower the shutter speed brighter it gets. i would suggest you to keep your shutter speed faster then 1/125 seconds at least

Aperture controls the intensity of your flash. 


keep this in mind and try :) happy clicking mate