To use Amazon Simple Storage Service, you need to register for AWS (Amazon Web Services) developer account and then subscribe to the S3 service.

Steps to subscribe to Amazon Simple Storage Service (S3)

Make sure you read all the instructions on this page and remember to "subscribe" to the S3 service after registering for a new AWS account with Amazon. Click on this link to go to Amazon S3's home: http://www.amazon.com/s3. You will see a "Sign-up" button towards the right on this page.

Follow the signup process after clicking on this link. You need an Amazon.com account, an AWS developer account and you need to Subscribe to the storage web service. Based on your current subscription levels with Amazon.com, you may see different signup wizards.

Somewhere during this registration process you will see a page as the one shown below. Select Amazon Simple Storage Service from the list of Amazon Web Services to subscribe to Amazon Simple Storage Service

Once you complete the registration process, you will receive an email from Amazon. This mail will contain the link which will take you to ‘AWS Access Identifiers' page , providing you the page containing your ‘Access Key ID’ and ‘Secret Access Key' .

If you see a message like this:

Important Messages

Your information could not be submitted because we encountered the following errors:

Usage reports are available for web services that are offered for a fee. At the present time, you only have access to web services that are offered free of charge.

Or a message like this: "Authorizing your account to access Amazon s3 service"

It just means that you are registered for the Web Services Account but not yet subscribed to the Storage Service . Just go to this page again, and click on "Sign-up for this Web Service" and follow the subscription process. This page provides you all the information related to Amazon S3 functionality, pricing, resources etc.

(http://www.amazon.com/s3)

Provide your password and the next page will show you the pricing details and ask for the payment mode for using S3 service. Fill up the payment mode and details required .Once it is done you will reach the page shown below.

Click on the ‘complete sign up button' to complete your subscription, and you will receive an email, once your S3 subscription is successfully done.

Related link:
What is my AWS Access and Secret Key?