If you want only authorized persons can use your tablet, then put him a password to unlock screen:
In the Home screen, press the "Menu> Settings> Location and Security> Define screen lock"
Select one of the options, for example PIN:
Enter a series of numbers and press "Continue"
Confirm PIN, then click "OK" and voila!
Do not forget to preserve the PIN in a safe place and that in the future to unlock the screen first have to enter PIN
Note: To receive calls you will not need to unlock the screen, but to perform them.
