There are lots of guides online, but often the details differ. For example, a recommendation to create a disk partition to separate OS files from non-OS files can be found in some guides, but not mentioned in other guides.
What are your recommended guides? They can be links, books or courses.
Also, opinions on Linux vs BSD when choosing a web server OS are appreciated (without any flame wars :-)
Use (centos) linux because learning how to harden it is a skill that will apply in many more places afterwards.