Here are a couple tips on taking better iPhone photos using the native camera app. These tips will help ensure you have correct exposure and a sharp subject.
Set focus: While the camera app does a pretty good job guessing on what you want to be in focus, it is not always correct. Simply tap on the screen where you want focus to be and the yellow square will show you your focus point.
Set exposure: Same with exposure you may need to set it manually by tapping on the screen where your main subject is that you want correctly exposed, then swipe up or down to brighten or darken the image to your liking.
Lock exposure/focus: You can also lock your focus and exposure so that if something is moving within the scene the focus or exposure doesn't keep changing before you have a chance to take your image. To do this tap and hold on your phone screen and the yellow box will show up along with AE/AF Lock (see image below of orchids). It will remain locked until you tap on another part of the screen.