QGIS is an open-source GIS software that is available to use for Windows, Mac, and Linux computers. It is a true desktop GIS that can perform advanced geospatial analysis and has an extensive library of plugins (>300).
For more information, see the QGIS website: https://qgis.org/en/site/.
A very useful resource to get started using QGIS is this website: https://www.qgistutorials.com/en/. This site offers a list of tutorials to become familiar with a variety of GIS operations using QGIS. The format of each tutorial takes users step-by-step through a given operation and oftens offers sample data to use for each tutorial.
The QGIS site has an official page on Training material. This include the following:
This introductory QGIS tutorial by GISGeography covers a variety of useful topics on how to use QGIS 3, including:
Brock University has made available this useful 6-page PDF document which gives a quick intro to QGIS primarily aimed at beginners. See the step-by-step guide here: https://brocku.ca/library/wp-content/uploads/sites/51/MDG-Intro-to-QGIS.pdf.
Duke University Library, "QGIS: Getting Started": https://guides.library.duke.edu/QGIS
Ryerson University, "Open Source GIS": https://library.ryerson.ca/gmdc/madar/gis/help/os/
The University of Tennessee Knoxville Library, "GIS Training and Tutorial Resources: QGIS": https://libguides.utk.edu/gistraining/qgis