The "sunny 16" rule is a handy guide. It is simple to remember, on a sunny day (assuming the subject is actually IN the sun) the exposure will be around f/16 @ 1/ISO
So, if you are shooting at ISO 200, you set the camera to 1/200th sec and f/16, or an equivalent exposure. Knowing those three parameters you can adjust each according to your requirements, if you want a wide aperture for less depth of field (say f/5.6) adjust the shutter speed by the same number of stops (in this case to 1/1600th).
In less sunny conditions alter accordingly, keeping shutter speed the same as the ISO, the aperture will be:
Sunny - f/16
Slightly Overcast - f/11
Overcast - f/8
Heavily Overcast - f/5.6
These are simply a guide however, as sunshine can vary in intensity depending on time of day, where you are etc. It is advisable to check the first shot's histogram on the camera, to ensure the exposure is correct.