If you need information about a particular program, you should first
man , or
There is lots of useful documentation in
/usr/share/doc as well. In particular,
/usr/share/doc/FAQ contain lots of interesting
information. To submit bugs, look at
/usr/share/doc/debian/bug*. To read about
Debian/Ubuntu-specific issues for particular programs, look at
Help on Ubuntu can be found on the Ubuntu web site. In particular, see the Ubuntu documentation pages for information on a wide variety of topics. The Ubuntu mailing lists and the Ubuntu Forums can be invaluable sources of help from the Ubuntu community.
A general source of information on GNU/Linux is the Linux Documentation Project. There you will find the HOWTOs and pointers to other very valuable information on parts of a GNU/Linux system.