Since you are interested in RHEL certification, CentOS
would be the logical starting point.
has a pretty good set of Linux pages.
has lots of useful resources, though they tend to be give more attention to the *buntus and derivatives thereof and to desktop users than to server administration.
offers much documentation and tutorials.
A web search for any particular topic, including the word "Linux" or the name of your distro is a great way to answer specific questions.
Whatever distro you choose to start with should also offer or at least point you towards documentation at their website.