To avail the AWS free tier, you need to add your credit card details. The card will not be charged for using services as per the free tier agreement, but there is change you might get charged for using services not within free tier or using more than the free tier limits. So it is a good idea to setup a billing alarm before getting hands dirty with AWS.
You can setup a billing alarm for some amount such as $2 or $5 so that even if you use some non free services accidentally, or forget to shut down some paid service with usage limit, you will come to know about it when the alarm mail comes, and can immediately shut down the service.
You need to create and setup AWS account first.
For setting up the billing service, you can use the cloud watch service:
Go to My Billing Dashboard (by clicking drop down arrow near to your name).
Under Alerts and Notifications, click on Enable Now for Monitor your estimated charges.
Once you select your preference, click on Manage Billing Alerts link, which will take you to cloudwatch.
Click on Billing and create an Alarm.