Manual and Help
Unix has thousands of pages of help information. If you forget how to use a command or need to use some unusual options you can refer to the built-in documentation for help.
The main help command is man (manual). Type man xxx to get help on the command xxx, for example man ls to get help on the ls (list directory) command. Press space at the end of each page or type Q to quit from the man display.
If you don't know the name of the command you need try the apropos command. For example if you can't remember the command to terminate processes type apropos terminate. You will see the kill command listed. You can now use man kill to get full details.
More information: Getting Started.