About xamarin and C#

Actually I have just completed the object oriented programming with c# and I just want to continue my programming career with c#. So I searched about xamarin and its tools and they are very effective I think. So I am at the stage where I cant able to think where to go. Obviously I am going for app development for windows android and ios, but I am really confused how it can be achieved by C#. what should I learn from this point to continue with c#. I completed just OOPS with c#. And I  am ready for the advancements. Please help me asap.