How to unlock my iPhone from turning sideways?

How do I unlock my iPhone from turning sideways?

On newest version (with multi-tasking)

1. Double click the home button to bring up the multitasking bar.
2. Swipe right until the last set of icons is displayed in the multitasking bar (these will include play/pause and skip buttons for iPod or Pandora).
3. The furthest left icon looks like a circular arrow - touch this icon to enable or disable rotation lock on the iPhone.
4. A rotation lock icon will appear in the status bar next to the battery charge icon when rotation lock is enabled. This looks like a circular arrow around a padlock.

Press the middle button twice and all your currently open apps will appear, slide to the right and you will see this round arrow.press that button and your done

Double click home button and go left to unlock portrait orientation