Hello, I'm trying to archive some of my messages outside my computer on a medium that I want to keep so that I can return to them easily. Is there a simple procedure suitable for computer ignoramuses like me :-) ?

The simplest way is to select the messages, then Ctrl+S to save them as eml files to an external folder. An eml file can be opened in TB and most other mail apps, and can also be imported back to TB by drag and drop to Local Folders.

There is an add-on, ImportExportTools NG, with additional options.

Thanks but I can't use this add-on. I download it and, when I launch it, it opens a new message in TB. No aditionnal fonctions appears Is it normal ?

See these instructions for installing add-ons:

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

This add-on places a button on the toolbar in the main window, and is also accessible from the right-click menu.