How do I rotate the screen on my phone

From the top right hand corner of your iPhone screen swipe down to open Control Center. Look for an icon with a lock in it. Is it red? Tap it and it will turn white. This will permit screen rotation. But note that not all apps or even certain activities on the phone can rotate.

FYI: Home Screen do not rotate.

Follow lobsterghost1 tip and test rotation with the Calculator, Notes or Photos app.