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.
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.
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.