Here are some quick summary notes on setting up a multisite on a WordPress installation. Before doing this yourself for the first time, I recommend doing extra research on the topic to be sure multisite is right for you and to be sure not mistakes are made during this process. As with every major change, do a full site backup first.

What is Multisite? wpbeginner.com has an article on this.

Add a line to your wp-config.php file:

define ('WP_ALLOW_MULTISITE',true);

that is added just above the comment that reads

That's all, stop editing! Happy blogging!

How do you do this? Use ftp. You can use the ftp utility in the Control Panel (cPanel) of your hosting company or as I do, use FileZilla. FileZilla is free and you can download it. Download the file to your computer, edit the file save the file and upload the file. I like to use Notepad++ to edit the file, but the options are many. Log in to the Admin dashboard and notice Tools Network Setup.

There are more steps involved as outlined in the link above at wpbeginner.com.