Once you log into Canvas and navigate to your course site, click Files in the left navigation menu. To start uploading files to your site, click the blue Upload button in the upper-right corner. You can also create folders to organize your files by clicking the +Folder button:
You can upload individual files, or groups of files & folders in a compressed .zip file (when uploading a .zip file, Canvas will ask if you just want to add the compressed file itself, or if you want to unpack and save the files/folders individually as they were compressed):
For more information on using Files in Canvas, refer to the Canvas Instructor Guide.