How to start using REST API?

It is our first time using REST API and after reading the official documentation we have some doubts. How can we execute an Atlassian command? Will we be able to substitute text using REST API, even though it will be in many different files, and separated in title and the rest of the content? Finally, will we need a specific Confluence user or just a normal Confluence user to be able to run the REST API extension?

Thank you.