Questions on building a simple API

I’ve got a pretty simple web application and would like to build an API so that firstly the iPhone and secondly other web apps can interface with it.

It only needs to do a few things - login, update, list records, refresh list of records (get new/updated only)

But I would like to do it right. Can anyone recommend best practices for doing this?

Should I be using REST, SOAP or XML/RPC?

Any info or guidance appreciated.