All you should have to do is sign up like normal, and chose the 2 Game Out Free Trial. You still have to give your credit card information, for a few reasons (If the games go "missing" or if your membership goes beyond the free trial).

The Free Trial is available only to GameFly first timers, so if you've signed up before, you won't be able to take advantage of it.

Also, if you don't want to continue your membership, be sure to end it before your Free Trial runs out or you'll be charged.