What is the best way to create back end for a mobile app as well as webapp? What platforms to use?

I want to create a webapp that works on pc browser as well as mobile app. I am amateur to the field of programming. Right now I have a vps on cent OS. with a very simple jsp webapp project (in the process of making jsp work on cent OS). But is there a better way to achieve this ?

Requirement : Backend to serve both mobile application as well as web application. What are the best platforms and choices to go for ?