When using flash, you will need to adjust your shutter speed to exposure for the background ambient light. The way you would adjust for the flash is with the aperture. If you are using the flash in ETTL 2, you won't have to adjust the aperture much. The Flash will increase power to compensate. Put your camera in manual mode. I would leave your aperture relatively wide open is you are shooting at night. You can close it down a little more during the day. Because the duration of the pulse of light coming from the flash is only about 1/10,000 of a second, your shutter speed will make no difference on the flash illuminated portion of the photo.
Adjust the shutter speed to absorb the ambient light from the background. You can get pretty sharp pictures handheld with relatively low shutter speeds. The flash makes the foreground nice and sharp. If you use a tripod, it will help to keep the background nice and sharp too. If you are shooting in the day time, you shouldn't have to use a tripod.
An example. at night: ON a tripod with ISO of 400, you would want an aperture of about 5.6 or lower and to get your ambient light, a shutter speed of anywhere from 1/30 of a second - 3 seconds. It all depends on how light out it is.
Test things out outside in the dark. The flash will expose for whatever you focus on that is close. You can set you f/5.6. Then take a bunch of photos while adjusting the Shutter speed down from 1/125 of a second. Notice the way the background or ambient light changes. This should help a lot.
I hope I am actually answering your question. I am sorry if this is stuff you already know.