How do I write a letter on my Mac?

Since you came to this forum from a link about using Pages, I'm going to assume you want to create a document, which you'll write as a letter?

This link will help you learn how to use Pages (which is an application, like Word is on a PC) --> Pages User Guide for Mac - Apple Support

https://www.grammarly.com/blog/how-to-write-a-letter/

Allan Jones wrote:

https://www.grammarly.com/blog/how-to-write-a-letter/

Certainly quite literal!