This guide provides information and specific steps for upgrading contributed and custom modules from Drupal 7 to Drupal 8. Drupal Module Upgrader can do some of this automatically for you, covering many API hooks. Check out the introduction and how to set up a development environment page for how to get started.
Source: Converting Drupal 7 modules to Drupal 8 | Drupal 8 guide on Drupal.org