Set a default text in an article
Couple options come to mind,

if you don't want to code then you can use Node Clone module and have 1 unpublished template node, and everytime you want to create a new one just clone the template node.

Also without installing any modules, you can use the Default value in the field setting of the content type

The other way is a bit more tricky, you have to create a custom module and use hook_form_alter to set the default values of the fields for your specific content type.