When I was using my ext flash in P mode, I couldn't adjust the aperture/shutter speed. my ISO was fixed at 100. how do I let the camera decide the power of the flash to match my own aperture, shutter speed, ISO setting?
Read here about flash with Canon: http://www.photonotes.org/articles/eos-flash/
In addition have your camera manual at hand to read there as well. Some details are not written as "If you want to adjust your shutter speed using 430..." but rather as footnotes in various chapters (different exposure modes, using flash etc).