How to determine what need to be Automate test's in a Web Application

I'm new to QA engineering.and I am clueless when it comes to determining what are the things I can automate when it comes to test automation in web applications.the applications, I'm focusing are JSP applications.it would be really helpful to get knowledge from an expert for me to get a better understanding how to use test automation in web application and what type of functionalities.

Thanks :)