How do I execute functions on a custom module?
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.