Place tissue paper or a white handkerchief over the flash to spread the light. Use a flash attachment to soften the light. A camera works by briefly admitting light through a lens and onto photosensitive film. F-stop and shutter speed jointly control how much light reaches the film for how long. Too much light and the negative is too dark.
Automatic cameras use a built in light meter to determine the best settings for both.
Professional photographers prefer to be able to set both to achieve special effects. You probably want your camera to handle these details for you. Shutter speed is simply an amount of time. To catch action without blurring, use a fast shutter speed. To get an arty blur, use a slower one, but put your camera on a tripod or other stationary surface.