How do I unlock portrait mode in iPhone? My screen has a lock with an almost full bordering circle around it ending with an arrow. This is located in the top right of my screen. Just inside the battery %. How do I turn this lock off so I can use landscape mode?
This is the lock rotation icon. You will notice that when you turn your iPhone (say, in Safari) it will stay in portrait.

To disable it:

On iOS 7+ : 


Open the Control Centre by sliding your finger from the bottom of the screen to the top
Tap the button a the top right (here circled in red)





iOS 6 


Double tap on home button
Then slide the multitask bar to the left (where the Sound controls are) 
And on the left you'll find the button to unlock this.