If you are going to develop the App in ASP.NET, C#, then I would suggest you to definitely study
ASP.NET AJAX[
^] as it will take a vital role in the App.
Because
AJAX
would help you update the page or part of the page without
PostBack
.
It will make your application more secure and responsive.
Learn about the
AJAX Control ToolKit[
^] and all the controls.
Also if you are familiar with
jQuery[
^], then it would make your App more responsive and eye-catchy.