To turn off your iPhone, follow the instructions below.
Step 1 - Hold down the sleep/wake button at the top right-hand corner of the iPhone (shown below) for a couple of seconds.
Step 2 - This will bring up a screen with a slider at the top that reads slide to power off. (Press cancel at the bottom of the screen to get out of the process)
Step 3 - To shut down the iPhone, move the slider to the right.
Step 4 - After this, a progress wheel will appear in the middle of the screen (shown below) for a few seconds until the iPhone turns off completely.