Welcome to APUG.
So here's a bit of a primer on flash.
The first thing to understand about flash is that it's range is limited, before everybody says well duh, that concept matters especially in fill flash.
I think of flash shots as two separate exposures; an ambient light exposure outside the range of the flash and then the main subject's exposure at a very specific range from the camera.
The ambient light exposure is controlled purely by the aperture, time, and film speed just like any normal shot. Just meter and set the camera normally to get your background where you want it.
The next thing to understand is that flash is really fast. A full power flash may last 1/2000th of a second, as power is reduced flash duration gets even shorter. Most modern flash guns control flash out put by controlling the duration of the flash. Since the flash duration is normally shorter than the shutter open duration, the shutter has no effect on the flash, only aperture and film speed do.
What that all means is simply that you set your flash gun to match the aperture and film speed being used in the camera.
With fill flash all we are trying to do is control contrast, a little pop to get the subject brightness more in balance with the background.
If the subject is covering a fair share of the frame, "auto" mode may work just fine. Dont know about your flash gun but my Nikon units will actually indicate the range of distances this method will work in once the aperture and ISO are set. If yours does the same, at this point you just go to shooting.
The manual mode takes a bit more work and practice and study. Essentially you dial in a power setting in the flash gun that will work at a certain range. If you move in closer you will need to reduce flash output, if you move away you need to increase output.