You must have a Google Checkout account associated with your Google Account to purchase items from Play Store.
On your computer, go to http://checkout.google.com to establish a Google Checkout account.
OR
The first time you use your phone to buy an item from Play Store, you’re prompted to enter your billing information to set up a Google Checkout account.
After you’ve used Google Checkout once to purchase an application from Play Store, the phone remembers your password, so you don’t need to enter it the next time. For this reason, you should secure your phone to prevent others from using it without your permission.