iOS will automatically change the orientation (portrait or landscape mode) your screen, depending on how the device is being held. Here's how to lock the screen orientation of the iPod Touch, iPhone or iPad.
Swipe your finger upward to display the Control Center.
Tap on the lock button located on the top right.
If you have an earlier version of iOS, it is still possible to lock the screen rotation:
Double tap on the Home button to display the multi-tasking tray.
Slide your finger from left to right until you see the screen lock button.