variable_get and variable_set in Drupal 8
variable_get()/variable_set() and variable_del() API was removed from Drupal 8 in favor of Configuration API.
So in order to save a simple variable in your module firstly you need to decide on a config object name and a name for the settings. Simple settings can be stored in a config object named <module_name>.settings, while for more complex settings it is advisable to use sub-keys.
To get a variable in Drupal 7 you would use:
$site_name = variable_get('site_name', '');
While in Drupal 8 instead of variable_get we use Drupal config:
$site_name = \Drupal::config('system.site')->get('name');
or if you are working within a context of a class:
$site_name = $this->config('system.site')->get('name');
To set a variable, in Drupal 7 was:
While in Drupal 8 instead of variable_set we use Drupal config:
\Drupal::config('mymodule.settings') ->set('mysettingname', 'myfirstvariable') ->save();