learn app and server interaction, recommendations please

Dear fellow developers,

I have a couple dozen apps in the app store, but they are all calculators and quick-reference guides -- everything contained within the app. But I'd like to learn new skills and capabilities.

I'd like to learn how to build an app which gets stuff (data, files, etc.) from a server. I've never tried this before. Can folks here recommend

• good resources to learn these skills and

• safe ways to learn these skills before trying to graduate to using corporate or university servers for interactions?

Thanks as always,

• - Josh