How to manage Classes (serialization / saving / query them)

Hi. I'm new to Xamarin and wanted to ask what would be the best way to manage classes. I had good experience with linq2sql with c#. Is this also possible for Xamarin.Forms with X-Platform?
I want to code a mobile chat app and a server application. So i need to be able to query the messages and sended files fast and i need to save them locally and on the server. What would be the best way to do so, regarding object-orientation, useability, serialization for submitting over the internet and so on?
Thanks in advance, Jan.