Glad Graham has sorted you out with a flash gun. If it comes with a Stofen-style diffuser cap, so much the better
If you're more comfortable on manual, then that's fine. Same difference.
Crash course on flash. All DSLRs have a focal plane shutter which has two blinds (curtains) which travel down over the sensor one after the other. They operate only at one fixed speed and different shutter speeds are produced by varying the time between each curtain setting off. The problem with flash comes at higher speeds when the second curtain needs to go before the first one has got to the bottom of the frame. If the flash is fired at these higher speeds, only part of the sensor is uncovered at any one time so the rest of the picture cannot receive any flash illumination. You get a big dark band right across the bottom of the picture.
The max x-sync speed, ie 1/200sec with your camera, is the fastest speed at which the whole of the sensor is uncovered for a very brief moment before the second curtain has to move. That's when the flash is fired, to synchronise perfectly with that 'very brief moment' when the whole image can be exposed. You can also use any longer shutter speed, but not faster. Obviously, the shutter speed itself has no effect on the brightness of the flash.
The flash duration itself is very brief, typically between 1/1000sec and maybe 1/20,000sec. The E-TTL metering adjusts the duration to control the power output for correct exposure and it calculates this by issuing a pre-flash which goes out as soon as you press the shutter release. The pre-flash is metered and that sets the power of the main flash which goes out when the shutter is fully open. This all happens so fast you cannot distinguish the two flashes with the naked eye.
Depending on which exposure mode you're in, the camera can also balance the flash exposure with the ambient light level. In this way, you can effectively get two exposures in one - ie correct shutter speed and aperture for the ambient light and then the flash output which will be balanced to match the aperture.
Things to note are that flash is very sensitive to distance, and follows the inverse square law which says when you double the distance, the light is reduced to one quarter, ie two stops. In practise this means that if you have subjects both close to the flash and some further away, the chances are that the near ones will be over exposed and the far ones will be very much darker, so try and arrange everything at about the same distance. The ISL also means that flash falls off quickly over distance, so keep things close or you'll run out of power.
When shooting outside using fill-in flash, there's a danger that the ambient exposure will want to push the shutter speed above the 1/200sec threshold. No worries, you can get around that by either raising the f/number or reducing the ISO (or both) so that the corresponding shutter speed is brought down under 1/200sec again. (High Speed Sync is a clever trick that some flash guns can do to sort that out another way, but you won't have that feature.)
Using the Stofen is great for bounce-fill indoors (it will do nothing outdoors, except waste light) providing the ceiling isn't too high. If you point it up at about 60 degrees, most of the flash is directed at the ceiling which gives nice soft bounce light, then there is also a small amount of light which goes forward direct to the subject which fills in shadows under eyes and chins, and adds a sparkle to the eyes. It's particularly handy for weddings when big hats are shading faces. If you don't have a Stofen, I think that gun has a pull-out bounce card which does a similar thing, and works well - see here
www.abetterbouncecard.com
E-TTL will do it's best to sort the exposure out in all these situations, and it generally does a pretty good job. If you don't like the result, on Av you can moderate the foregound exposure by adjusting the power of the flash (the gun should have a compensation control for this) and you can adjust the brightness of the background ambient exposure with +/- compensation on the camera, which will adjust the shutter speed accordingly.
I'm in Aberdeenshire though, bit far....
