Taken (non literally) from the LFS201 course intro:
Take the edX Introduction to Linux LFS101x course. This is free and unlimited and sets a good base from where to start form
Topics which are not included but necessary are knowledge of:
text editors such as vi, emacs, nano, gedit, ed (at least one for shell and one for GUI)
linux documentation (CentOS can't publish documentation anymore, so if doing CentOS use RHEL's documentation)
manipulating text with sed, grep, awk, cut, paste, tail, head, etc
file utilities such as find, locale, file
Printing
graphical interfaces and their administration
bash shell scripting
system installation
Link to a PDF with the LFCS competencies (easier to print out) version 2.16
You can download the Redhat system administration manual as an epub or view online.