For the Red Hat distributions and derivatives, you can get useful info from the guides and manuals at Red Hat and the Fedora Project. You can also get help from other users on the RedHat mailing lists. Note: they are populated by volunteer users, not necessarily Red Hat support personnel.
Your best overall source is The Linux Documentation Project wherein you will find the HOWTOs, Guides, and other goodies.
Another good source is the Linux Administration Made Easy page.
For specific commands, you can look at the "man page" (e.g. "man ls"), or the "info" page such as "info chmod". The info pages are the putative successor to the man pages.
I have some wisdom to impart on some subjects, based on my experience: