When you register you Azure account you need a Windows Live ID and a Credit Card and you have to register your credit card information and the usage will be charged on the credit card.
But wait a minute, I want to use the 45 day trial or the MSDN subscription to develop and learn SQL Azure. Well, don’t worry, prior the the December 2011 Service Release, you got charged for the usage that wasn’t included in your Azure subscription, but after the release you can’t use more than whats included unless you change your settings. If you are going to sign up for the ”real” Azure account you need to specify the price plan:
The most flexible subscription is the Pay as you go and you can increase / decrease the number of databases etc and the charge will drop accordingly. The 6 Month subscription is less flexible, but you get a discount. During this period you can increase the number of databases, but you can’t decreas until the end of the period. You should choose the subscription based on how you are going to use the platform. If you need to scale up and scale down during a subscription period, you should use the pay as you go subscription.