On most smartphones, tapping on the screen will tell the phone where to focus. However, if tapping the brightest part of the photo takes the main subject out of focus, you still have another option.
With your camera app open, tap on the main subject and hold for a few seconds. Typically, an exposure slider will pop up on the display, allowing you to adjust the overall brightness.
Depending on the phone orientation, make, and model, swipe left or down to reduce the overall brightness of the photo and increase the depth of your shadows.
If neither method works, you may have to enter a Pro mode or, otherwise, access additional settings for the camera on your phone. From there, adjust the ISO to change the brightness of the photo you're capturing.
Bonus: Side Lighting
Through the use of light and shadow, you can transform even the most mundane subjects and locations into dramatic photos. You can experiment with shadow by using side lighting to bring out deep shadows and emphasize different textures.