What you are describing is a difficult problem for beginners using on camera flash. Strobe lighting typically used by pros and more advanced amateurs typically have "modelling light" built in. These are incandescent bulbs that are on all the time and let the photographer see where the light is going. We also don't use the hard flash/strobe of a on camera flash because as I described before you get very harsh light and harsh shadows.

As was previously mentioned by glbeas you can use a reflector card/or sheet of some sort, to bounce the flash off of or bounce the flash off the ceiling (which I don't really like because of the angle of the light).

What I would suggest is to try available light. You will need a tripod and will have a fairly slow shutter speed. Faster film will help. Buy a piece of fomecore, white card, or a sheet, and use this to bounce some of the window light back onto the shadow side of the subjects face. Then the light you see is the light you get.

What you were attempting is a pretty difficult thing to achieve. You were attempting to have a flash fill to even out the available light and doing this with a camera strobe is not easy. What happened is your flash was way brighter than the available light and overpowered it and in the process produced the harsh shadow on the background. One thing that you could do it to take the flash off the camera, and have it a few feet to either side of the subject and above them a little so the shadow will fall lower behind their body and not right behind their head.

Hope this helps.