How do I get my phone to change from portrait to landscape when I turn my iPhone?

Note that not all apps can rotate, but ensure your phone will rotate those which can, from the top right hand corner of your iPhone swipe down to open Control Center. Look for an icon with an A in it. If it is red inside a white box, tap it and you will have unlocked rotation.

Check in the Control Center if you have Screen Orientation lock turned on:

Change or lock the screen orientation on iPhone - Apple Support

Not all apps will support screen rotation. You will not see it rotate on your Home page, but if you open the Photos app you should see the screen rotate.