RESTful webservices can support all of the HTTP methods.

I am not certain I fully understand the other part of your question: "which will be the first function to be called"

Functions/methods are called as they are required. On thing to remember though is that methods generally don't have an order in the sense that one is waiting for another to complete. REST is built around the concept of statelessness, meaning that no method can know what came before it. It simply sends it's information and receives a response.

Hope that helps some

Also, there is no requirement in REST to use XML.