How do you get World of Warcraft 30 day trial for free in war of Warcraft

There is no 30 day trial- there is a free 10 day trial (during which you can level to 20, but cant trade nor talk in general channels). You can sign up for the 10 day free trial at the WoW site https://us.battle.net/account/creation/wow/signup/
What you are probably thinking of is the free 30 days of game time you get after you purchase the retail copy of the game.