Follow the one-a-day plan. Start by listing everything in
/sbin. Then take one command per day and type
man command and find out what it does. The essential core commands are all in
/sbin. After that, go to work on
/bin and when that is finished, go to work on
/usr/bin. You can shorten your list of commands by skipping any with more than 5 letters in the command name, and you will not miss many important things.
But the important thing is that you will learn something new every day, and probably before you are half way through, you will understand so much of the basics, that you can easily pick up half a dozen new commands per day.
But you have to use them, each and every one of them, even if it is only tinkering and doing exercises with them.