Reddit mentions of Mercator : The Man Who Mapped the Planet

ESRI products are great and you can steal them online, but if you're going to get education in GIS stuff at a University or IT then you will learn that stuff there. QGIS is a free platform that is a good place to start, but honestly if you can fully explore Google Earth capabilities that might be more applicable as many companies and organizations use it to some degree. Whatever free GIS platform you choose, just go through every tool and understand how to use it, you will be good to go.

That said, just read a book and learn about geographic and cartographic principles. Maybe look into what area of GIS you are interested in, like environmental modelling or more civil stuff.

Learn about remote sensing and the capabilities of the specifications on the satellites, drones, cameras, sensors, and other equipment out there.

I have worked in GIS for over 7 years, working for hydrologists, geologists, wildlife biologists, engineers, and city planners. It is a fun field that is ever-evolving. It is important to have some advanced computer skills, especially regarding spreadsheets and databases. Understanding VBA SQL and Python are helpful, but I only know a bit of each. I do not like developing tools and writing scripts, so I don't. I use model-builder to get the most out of available tools. Get a solid geography background too, I don't mean name all the rivers in Europe, people can look that up in 5 seconds, learn the principles and understand projections and how maps are used and how surveys work... If you are going to higher Ed you will learn most this anyway.

Try making maps for something you like, like a hiking trail or your back yard, improve them, share them, see what people think. If you have a smartphone, try some GPS and GIS apps, go collect some data points, lines, or areas and try to make a map. Maybe even use a GIS or a drawing app for the map (but a desktop or laptop computer is easier to work with for that). Mobile technology is an exploding realm within GIS these days. It is important for me to be able to send non-GIS people into the field with a smartphone or tablet and ask them to collect data with many attributes. have them correct and QA the data, then upload it onto a website that will automatically update a web map.

If you search online a bit you will find more than enough material on GIS and free GIS platforms to fill your days with. You can learn lots in school, but they leave a lot out too. Having your own projects is not only fun it will give you an advantage in the job market.

Edit: I didn't edit anything except this line. I apologize for my abysmal grammar and structure.