How do I execute functions on a custom module? I am writing my first custom module and would like to test its functionality, but am not certain how to call functions to run them from within Drupal.  The module is installed.
You can call specific functions for testing using drush (drush ev "command();") or using the devel.module block/page (at /devel/php or so).

However, apart from some quick tests, in 99% of the cases, Drupal is going to take care of calling the functions of your module, through hooks and callbacks.

E.g., to have your module display something in your site, you need to implement hook_menu(), define a page callback then simply visit the defined path.