I'll try to help you. First, ISO is written on the film cartridge. Put a film in the camera and ISO is fixed for the duration of that film. Set the ISO on your camera to the same value (if your camera has a light meter) and then stop thinking of it for the time being.

I think it will be easier to learn if you skip the flash for now.

I am not familiar with your camera. Can you put it in manual regime? Then do that. Then ask yourself how much light hits the film. You can either open up the aperture or make it smaller. This is one variable. You can also increase or decrease the shutter time and this is the other variable. The apertures and the different shutter speeds are set so that each full step (or "stop" as it is called) either doubles or halves the film's exposure to light. So if you decrease shutter time one stop, and open up the aperture one stop, the film's exposure to light remains the same. Works in both directions and for any number of stops.

Now, your camera's light meter might be broken. Or your film was old and had expired. Or the development of the film was not done well. Or your flash did not work. First check your camera's light meter to another camera. Digital is ok, you can see if its in the ballpark.

It might also be bad scanning of properly developed negatives.