Hello. I would like to pre-define a page in Confluence. I only want to start with that page and can just change things in it. Only copy&paste stuff. Right now my problem is that I copy&paste right now and lots of formatting errors occur. 
Yes, that’s possible with page templates. You can see the Atlassian documentation on how to work with those here:
https://confluence.atlassian.com/doc/page-templates-296093785.html
Here you can learn how to make your own templates: Create a Template | Confluence Data Center and Server 8.0 | Atlassian Documentation
