How to disable cache for a custom page ?

Disable cache for a custom page from route declaration.

If you want to disable cache for a custom controller (Custom module), You have the no_cache option (YOUR_MODULE.routing.yml).
Example :
File: mymodule.routing.yml

mymodule.myroute:
  path: '/mymodule/mypage'
  defaults:
    _controller: '\Drupal\mymodule\Controller\Pages::mypage'
    _title: 'No cache page'
  requirements:
    _access: 'TRUE'
  options:
    no_cache: 'TRUE'