There are hell lot of resources available on web which will help you porting from D7 to D8. There are lot of changes in Drupal 8. Drupal 8 has borrowed some components from symfony. Read following articles about drupal and symfony.
1. http://drupalwatchdog.com/volume-4/issue-1/something-borrowed-something-drupal
I suppose porting module will be a great way to understand drupal 8. So i tried to porting a module name username_check to drupal 8. Here is the github repo for D8 module and the presentation i created to record all the changes involved in porting the module to D8.
Here are the few resources which i collected and few which i created.which will help you to learn porting.
1. https://www.drupal.org/update/modules/7/8
2. https://drupalize.me/blog/201412/adventures-porting-d7-form-module-drupal-8
3. http://drupalwatchdog.com/volume-4/issue-1/upgrading-your-modules
There are more available on web. Since D8 is still in beta so you can’t just stick to these blogpost or article, you need to keep exploring.
