How to get the fields of the entity? I need to get all the names of the entity, my entity is called 'a_process'? 
How can I write such query in Drupal 8? 

$nodes = entity_load_multiple('a_process', $name);
You can use the entity type manager to load entities of any type:

$entities = \Drupal::entityTypeManager()
  -&gt;getStorage('a_process')
  -&gt;loadMultiple();

foreach ($entities as $entity) {
  // ...
}


If your code is in a container aware context, be sure to inject the entity manager as a service instead of referring to \Drupal explicitly.

Of course if you always know the entity type up-front you can use the method on the entity class itself, eg

$entities = \Drupal\foo\Entity\AProcess::loadMultiple()