Street scenes require a photographer to work fast or potentially lose the moment. Unlike a posed portrait, there are no re-dos. ❤️ There is rarely time to fish the camera from the bag, remove the lens cap and fuss with your settings. Considering how unpredictable life can be, it's best to prepare for anything.
I start by dialing in all of my camera setting based on the existing light. The precise settings will vary depending on the available light, but here are the main technical considerations.
1) Shoot with a fairly small aperture so you have enough depth of field to keep your subject(s) in sharp focus. As the old war photojournalists would say, "f8 and be there", meaning put yourself in the right place at the right time, set the camera to f8, and you have a good chance to succeed.
2) Select a shutter speed of at least 1/250 which is fast enough to freeze a person walking and also prevent any camera shake. If your subjects are moving very fast, you may even opt for 1/320 or 1/500.
3) Since you'll be using a small aperture and fast shutter speed, a high ISO will likely be required. Just how high should it be? Try 800 first and go to 1600 if the photo is still too dark. The ISO will make your camera more sensitive to the existing light, absorbing it faster and brightening up the scene.