I am developing an application for iOS and that is going smoothly, the problem is that I am very new at server sided things. I am totally confused about how to correctly use Amazon Web Services for this purpose.

What I want to do is very simple. I want my application to be able to query a servlet hosted in EC2 to be able to retrieve pictures and data based on some criteria from S3 and SImpleDB respectively. Also the application should be able to upload pictures into a S3 bucket and register the information in the SImpleDB.

My main concerns are security and costs, So far i was using Amazon Token Vending Machine but I haven't been successful when trying to customize it, and while researching I discovered that on the long run it is very expensive.

The ultimate goal is to handle a "social" picture service for my iOS application. Being able to register new users, authenticate these users. See what permissions they have to which pictures from the bucked. And all this without having to worry about Third party people from accessing the private pictures of my users.

Sorry for this question but I am really clueless about how to handle this... I have tried reading many articles but all these server stuff looks very scary.