1: Uninstall whatsapp,if your trial version is finished, or you want
to extend it for a year.
2: Go to gmail application and
create a new gmail id.
3: Once you have made it,wait for some time (it totally depends on
your connection speed,ideally you
should wait for 15-20 minutes)
4: Go to google play, click on the menu button of your android and choose "Account Settings".
5: Once you have done this,you just need to choose the new gmail
account, you just created, and download whatsapp once again.
6: Now open whatsapp, and sign into your whatsapp account...
Note:- This way you will make it free for one year,to make it free for
lifetime, you will have to apply this trick once every year.