When you begin the free trial, Google creates a billing account for you and credits $300 to your account. Anything you do that would normally result in a charge is billed against this credit. You will receive billing statements indicating how much of your free credit has been spent.
For information about what happens when you spend all of your free credit and how to upgrade to a paid account, see Billing during the free trial and Billing after the free trial.
Note: You should be aware that if you do not upgrade to a paid account after the free trial ends, your Google Compute Engine resources might be removed. Removed resources are not recoverable.