If you need information about a particular program, you should first try man program, or info program.

There is lots of useful documentation in /usr/share/doc as well. In particular, /usr/share/doc/HOWTO and /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 /usr/share/doc/(package name)/README.Debian.

The Debian web site contains a large quantity of documentation about Debian. In particular, see the Debian GNU/Linux FAQ and the Debian Reference. An index of more Debian documentation is available from the Debian Documentation Project. The Debian community is self-supporting; to subscribe to one or more of the Debian mailing lists, see the Mail List Subscription page. Last, but not least, the Debian Mailing List Archives contain a wealth of information on Debian.

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.