Index.php isn’t for a specific page, and that’s not how template files in themes work.
You will need to understand the template heirarchy
http://codex.wordpress.org/Template_Hierarchy
WordPress uses the Query String — information contained within each
link on your web site — to decide which template or set of templates
will be used to display the page.First, WordPress matches every Query String to query types — i.e. it
decides what type of page (a search page, a category page, the home
page etc.) is being requested.Templates are then chosen — and web page content is generated — in the
order suggested by the WordPress Template hierarchy, depending upon
what templates are available in a particular WordPress Theme.WordPress looks for template files with specific names in the current
Theme’s directory and uses the first matching template file listed
under the appropriate query section below.With the exception of the basic index.php template file, Theme
developers can choose whether they want to implement a particular
template file or not. If WordPress cannot find a template file with a
matching name, it skips down to the next file name in the hierarchy.
If WordPress cannot find any matching template file, index.php (the
Theme’s home page template file) will be used.