Here's a simple example of how to kill ambient light (perhaps from a distracting background) and light the subject with off-camera flash, triggered optically.
First, our control test - meet "Ted" (given to me by my Grandmother, nigh on 50 years ago):
Personally, I do not rate either photo, especially the background, so lets get rid of it...
All photos are straight out of the camera (no post processing)
Canon 7D with the popup flash triggering a Yongnou YN568EX flash in slave mode.