1. if you know the command name X; from the shell
2. if you need more details try manpages
or google: "man X" to get it in a friendly format
3. if you want to know which commands are doing X.
4. more than man, try info
5. Google is you friend
6. The Linux Documentation Project
http://www.tldp.org/
7. read a book! believe me: it's not a waste