You can use the sunny 16 rule... for example using ISO100 film on a sunny day set shutter speed to 100 (or nearest) and aperture to f/16. Therefore with the same rule, for ISO400 you would set speed 400 at f/16... it's a simple and very usable system and there are more in depth details via Google but basically you can go from the sunny @ f/16 to the cloudy @ f/8 to the inbetween @ f/11 and the downright dismal @ f/5 on the same shutter speed...
You can use the reciprocal of the film speed to drop a stop every 50%... so assuming ISO100 at 1/125 (nearest to 1/100) at f/16, for going down a stop to f/22 you need to half the shutter to 1/60... conversely to go up to f/11 you need to double the shutter to 1/250... get the idea? That lets you figure for dof considerations.
Arthur