If you're using the AWS console, then you can associate your desired hostname (e.g., www.example.com) with the IP address of the EC2 instance using a CNAME record to the external DNS name for the instance.
For example:
Navigate to the hosted zone you created (double click works)
Click [Create Record Set] at top
Name: www.example.com
Type: CNAME
Value: ec2-xxx-xxx-xxx-xxx.compute-1.amazonaws.com.
Click [Create Record Set] at the bottom of the form
I also recommend you learn about Elastic IP addresses. You should allocate an elastic IP address, associate it with your instance, and then map the CNAME to the new external DNS name for the Elastic IP address on your instance. This will let you move the Elastic IP address between instances without having to update your DNS.
If you're curious why I recommend a CNAME instead of using an A record, here's a benefit I wrote about when talking between instances inside EC2: http://alestic.com/2009/06/ec2-elastic-ip-internal