X-370S 35mm SLR Camera: what is the best aperture, shutter speed and iso f...
Your camera light meter should tell you, but in case it doesn't work then all you need to remember is the "Sunny 16 Rule". Basically, on a bright day and with the lens aperture set to f16 you set the reciprocal of the film speed.
So with ISO 100 film the reciprocal is 1/100, or 1/100th of a second. Your camera shutter speed dial doesn't have that, but it does have 1/125th which is close enough.
Once you have the exposure set for f16 then if you adjust the aperture you simply adjust the shutter speed to match. So if you decide to set f11 that allows twice as much light onto the film so to compensate you reset the shutter speed to 1/250th of a second to halve the light coming in. As twice the light coming in through the aperture is compensated for by halving the light coming in via the shutter, the exposure remains the same.
Although this works for a sunny day, you just guess based on experience other lighting conditions. So if it's bright but overcast then you can leave things as they are, but if it's dull and overcast allow one extra stop of exposure by opening the aperture by one setting or by doubling the time that the shutter remains open. So from the starting point of f16 at 1/125 (as above), on a dull cloudy day you'd either set f11 OR 1/60. If you set f11 AND 1/60 then you'll be allowing not just twice as much light in but three times as much.
Hope this helps, if so please take a moment to rate my answer. If it's too complicated then please add a comment and I'll re-explain in even simpler terms.