Hi Anand,
The main file you'll want to take a look at will probably be
homeSuccess.php. Keep in mind that the header and footer of the site are separate files, and some other elements may be components managed in separate files.
A better place to start will be some of our resources that will help you understand how AtoM code is organized, and how theming works in AtoM and Symfony, the underlying PHP framework used.
The following forum thread has a lot of additional links and context that should help, including links to our official theming documentation and to a slide deck on developing custom themes:
If you do some searching, you will also find many other previous threads in the forum with useful information. For example, this results page will show all threads that have been tagged with both "themes" and "FAQ" as labels:
Hope this helps!