Find a project and do it. Sounds easy right?
Here's the easiest way
1) buy old laptop or desktop - this way if you trash it , you can always start from scratch. Under $100 for something old.
2) Install linux on it. Ubuntu is REALLY easy and almost does it all for you. I would say try CentOS because that's more like you'd see in a real business
For a project:
3) learn to put a static ip on it
4) put apache on it and get it to be a webserver
5) write simple HTML to do "HELLO WORLD"
6) figure out how to open browser and point to your webserver and display "HELLO WORLD"
Hello World is the beginning of all things
7) install php. Write PHP file to display "HELLO WORLD"
8) install mysql. Create a database with one table with one column with the string "hello world" and write php script to pull from DB and display contents of that column
After you've done all that, you have a pretty good grasp of all things Sysadmin related with overlaps into programming, DB, and networking