Help:MultiBoilerplate
When creating a new wiki page, you can select a page template in the "Select boilerplate" dropdown.
The list of available page templates for the main namespace is managed in Mediawiki:Multiboilerplate.
Templates for other namespace are managed in Mediawiki:Multiboilerplate-<namespacenumber>. To find out the namespace number, go to a page in that namespace, view the page source and search for the <body> tag. It includes a ns-<namespacenumber> class - for example, this help page has ns-12.
Note that only wiki admins can edit the list of available templates.