Setting the description used by new blogs

You get that information calling get_bloginfo('description')'. So you might code a custom function to set such data.

API: http://codex.wordpress.org/Function_Reference/get_bloginfo

WordPress is based on Hooks, Actions and Filters. You can get the big picture on its API docs:

http://codex.wordpress.org/Plugin_API

On this case you might code an action triggered when a site is created, or when the theme for the new site is setup. But you already have examples using filters:

http://wordpress.stackexchange.com/questions/33522/changing-bloginfo-description-from-a-plugin