You could just rely on the automated exposure/metering system of the camera, and put the flash on
Automatic mode, so that it will turn on the flash when it thinks it's needed, and you don't have to worry about it.
Given that your camera is a film point-and-shoot with a fixed 35/2.8 lens, and is fully automatic without giving you any explicit control over ISO, aperture, or shutter speed, it may not be worth your while to, say, put a metering app on a smartphone and see how much light you have to determine whether or not to turn on the flash. :)
As for which type of film to get, the ISO rating is mostly what you have to worry about, and it's a tradeoff between low-light sensitivity vs. grain in the final shots. The higher the ISO, the more sensitive the film is to light, but the larger the film grains are.