It is very tough to predict the kind of back-end you would need without knowing the inside out of what you are building. Even after knowing all those, you will have to change things on the fly.
I would suggest you get started, then measure and improve on the fly. I can just give you some guiding points, though:
- Try this service called Elastic Beanstalk with m1.small instance to start with. This comes with an ELB and I would suggest use it.
- Store large files like video and sound on S3 (EBS is not for such needs)
- Database - If you have rich query needs, then use RDS (otherwise consider DynamoDB - Dynamo is easy to manage and it gives infinite scale).
Another good resource for you is the AWS reference architecture case studies: Architecture Center : http://aws.amazon.com/architecture/