Browser based, but adjusts to device used

First, read this with the understanding that I am biz owner tickering with ideas and learning to write code where our main biz app is a custom app that is fully functioning at this time.

What is a good platform to start with where the intention is to allow clients to log onto and manage their page?  Our custom biz app uses IE8 as the interface, SQL as the backend, now wondering which program to use to develop a page the clients can log in from and would like that page to be automatically adjusting to their device, ie, from an iPhone vs from a laptop, the page they are logging in from automatically scales to their screen view size.

Here is the list of tech used in our custome biz app:

C#
ASP.NET 3.5
JavaScript
AJAX
Ajax.NET (http://www.ajaxpro.info/)
WebDAV (Exchange interfacing)
SQL Server 2008

Libraries
iText (PDF generation)
zxing (barcodes)
jQuery

Version Control
Subversion SVN