There are a lot of resources to learn Drupal, with a wide range of prices and time commitment.
The first step for everyone should be to create an account on Drupal.org (and optionally -- but highly recommended -- join the Drupal Association).
If you're responsible for maintaining sites, you should add a subscription to the Drupal security newsletter (you'll get notices every Wednesday of security issues with Drupal core and modules). Even if you're not doing the maintenance, you can be aware of what issues are being fixed.