Actually, you don't start with the REST step to define what you need, rather, you start with your Web Services API provider to figure out what is required.
Do you know how the API is to be used? Do you have documentation or examples to reference from your API provider?
Every API is a little different, so there's not a one-size-fits-all answer here. E.g., One API might use POST to update a record, while another uses PUT. One API might look for the data to be in the URL, while another looks for it in the payload. One might use JSON, another might use XML.
