If you are looking for some help on a particular subject or command, man pages are a good place to start. You normally access a man page with man <command>, but you can also search the man page descriptions for a particular keyword. As an example, search for man pages that discuss logins:
man -k login
When you access a man page, you can also use the forward slash key to search for a particular word within the man page itself. Simply press / on your keyboard and then type in the search term.