Start with these tried-and-tested settings for natural light or flash-lit headshots
When the subject is moving (as people almost always are), you’re restricted to fast shutter speeds. Here’s a good stock set-up for outdoor headshots: Manual mode, Auto ISO, a shutter speed of 1/250 sec and your lens’s widest aperture setting, such as f/4. With flash, use similar settings but with ISO100. Take test shots to work out the best pairing of aperture and flash power.