Forget Av, Tv and P modes. Use manual settings for the camera. Set your lens to use the widest aperture possible and set the focal length towards the long end of the zoom. 55mm and f/5.6 should be fine.
Your choice of shutter speed and ISO will depend upon how much ambient light you want to include in the scene, relative to the strength of the flash. If the ambient light is fairly bright indoors then try 100 ISO and 1/200 as your shutter speed. Let the flash take care of its exposure in automatic ETTL mode.
If the background is too dim then try slowing the shutter speed down to maybe 1/100 or 1/60. That will brighten the background without affecting the strength of your flash. If the background is still too dim then increase the ISO to maybe 200 ISO or 400 ISO. This will also affect the flash exposure but the flash should adjust automatically to compensate.
If the subject illumination by the flash is too weak or too strong then use Flash Exposure Compensation (FEC) to adjust the strength of the flash. Don't be at all surprised if you have to dial in maybe anything between +/- 1 stop of FEC to get the result you want.
You will get more pleasant lighting from the flash if you can bounce the light off a white wall or ceiling rather than aiming it directly at the subject.
It's hard to be more precise without knowing the ambient lighting conditions and the distance to your subject and to the background. If you want any more suggestions it would be helpful if you would post an example picture with the EXIF data left intact.
Here's an example of a snapshot/portrait taken with settings as follows...
40D, 95mm, 100 ISO, f/5.6, 1/250, diffused (I think) flash..
It's not great, or arty, and not tweaked, just my girlfriend standing in the doorway leading out to the hall, but it illustrates the results from settings similar to those I suggested.
