To store user uploaded files on amazon s3 you will need to create a new bucket and enter credentials for it into your site "admin > settings > uploading" page.
Create a new AWS account.
Open https://s3.console.aws.amazon.com/s3/bucket/create url, enter the bucket name, under "Object ownership", select "ACLs enabled" and "Object writer". Leave all other settings set to default and click "Create bucket" at the bottom.
Click "Security credentials" in the top right menu and then "create access key" button.
Copy the "Access key" and "Secret access key" from the next page and paste them into settings page on your site, along with your bucket name and region.