how do i change the orientation of my ipad screen?

It should change when you turn the iPad (no action on your part needed), unless you have it locked (check teh switch on the side of the iPad (just above the volume rocker); or if you have that switch set to control Mute, the orientation lock will be in the app tray (double-click the home button and swipe to the right)

For IOS 7 or later you will have to swipe up from the bottom of the screen and tap on the lock.

Thanks! Easy fix.