How can I prevent taking blurry photos

See the "Your photos are blurred or out of focus" section in:

Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support

You can try holding down on your camera screen (in the camera app) and it should turn on AE/AF LOCK (auto exposure, auto lock) indicated by the "AE/AF LOCK. You can also adjust the lighting with the slider (indicated by the yellow sun icon [if you have flash on or if auto flash is on, the flash icon will be next to the AE/AF LOCK icon]). This allows you to lock the focus and lock the exposure of the camera. This is good for when you know you are going to be at a constant distance from the subject you are taking a picture of, having trouble focusing on the subject, or having trouble with getting the desired lighting.