How to Lock your iPad (More in iOS 6)

Don't like your kids to mess with your device or someone to read your emails on your iPad? Lock it with a password.

Open the app Settings and select "General" on the left panel.
Select "Passcode Lock" on the right panel.
Tap "Turn Passcode On".
Enter a 4-digit passcode, and the same again to confirm.

Each time when your iPad is turned on, you'll have to enter the 4-digit passcode to access your iPad.

In iOS 6, you can lock your iPad to a certain screen of an app you choose, or disable touch access to certain areas of that screen. To enable this feature, turn on Guided Access as follows:

Open the app Settings and select "General" on the left panel.
On the right panel, select "Accessibility", then turn on "Guided Access".
Tap "Set Passcode" and enter a 4-digit passcode twice to confirm.

To lock your iPad to a certain screen of an app:

Open an app and select a screen you want to lock, triple-click the Home button.
Turn off "Touch" to lock the whole screen, or circle areas on the screen you would like to disable.
Tap "Start" to begin with Guided Access.

To end Guided Access, triple-click the Home button, enter the passcode then tap "End".