Well, was most of that $500 for storage costs, or for network? Surely you don't expect the monthly bill to include details about particular files or bucket sizes? Have you enabled CloudTrail logging and/or VPC flow logs?

It's not my intention to victim-blame here, yet do bear in mind that there are many, many ways to use the services and to expose resources to the bottomless appetite of the internet. Maybe Digital Ocean or Linode or the like would be a better option for starting out.