Darkening at the edges is a result of your lens having a wider angle of view than the flash has. Probably happens mostly when your lens is at 28mm. A larger diffuser may help this, or try not to go wider than 35mm when using your flash.
I've never found an auto-exposure mode that does a good job with flash; I'm sure folks have them, it's just never worked well with any of the equipment I've used. Start by figuring out your flash's guide number (GN), and then figure out how to set it on full power manual. The guide number is usually listed at ISO 100 and in feet or meters. GN divided by distance to subject equals the f/# for "correct" flash exposure. So if your GN is 100, and your subject is 10' away, with ISO 100 film you would choose f/10 (or the closest to it you can get since that's an odd f/#, for older cameras it would be the halfway setting between f/8 and f/11). Try this out see how it works. You may find that you prefer to over or under expose, but now that you are running the camera you can experiment, and see what works for you.
Frontal lighting tends to suck (IMHO). Get a cord so you can get off the flash off the camera some, or try bouncing the flash off the ceiling.
Keep trying and take notes. For me using flash for lighting has been by far the hardest to learn aspect of photography.