How To Add a List or Library to Your Site

A SharePoint Online site comes with one default library, however you can create as many libraries as you need provided you have the right level of access (Edit and above). You can also add lists or other apps to your site.

Instructions

From your Modern homepage, select 'New' from the menu bar

Then chose what you want to add to your site and provide the details when prompted.

You can also complete these steps from the Site Contents page (Gear--Site Contents)

Add Lists and Libraries To Your Site

How To Add a List or Library to Your Site

Instructions