I want to create a custom login but I don't know where to start,
See the image for more details:
Is there anyway to create a custom HTML and add it the drupal login logic? maybe with a REST or API?
Here is a very good tutorial on Customising the login form.
I got what you need but please mention which Drupal version you are using; you can have a look at this module if you need: It is one of the best module for customizing the login of a Drupal system in the Drupal way.