Tap, hold and slide: focus and exposure
The camera makes two guesses every time: what to focus on, and how bright to make the picture. Both guesses are easy to overrule, and overruling them is where sharper, better-exposed photos come from.
Tap to focus
Tap the screen on the thing that matters — usually your subject's eyes, or the nearest important detail. The phone locks focus there instead of on whatever happened to be in the middle. This alone fixes a lot of "almost sharp" photos.
Slide to set brightness
After you tap, most cameras show a small sun or slider beside the focus box. Drag it down to darken the photo, up to brighten it. This is exposure, and you want it in your hands, not the phone's.
When should you darken? When the sky or a window is blowing out to pure white, pull the brightness down until you can see detail in the bright areas again. When should you brighten? When a face is in shadow and coming out too dark. You are deciding which part of the scene matters most and exposing for that.
Hold to lock
Press and hold on the screen for a second and the camera locks focus and exposure (often shown as "AE/AF Lock"). Now you can recompose — move the phone to frame the shot how you like — and the settings stay put. Tap once more to release. This is handy when your subject is off to the side.
Keep it steady
Blur is often just shake, not bad focus. Tuck your elbows against your body, breathe out, and press gently — or use the volume button, or the timer, so the tap itself does not jog the phone. In dim light this matters even more.
One thing to try today
Shoot a person standing in front of a bright window. Let the phone decide first — the face will be a dark silhouette. Now tap the face and slide the brightness up. Same scene, completely different photo.