Documentation on QuickTime Streaming Server in pdf format is available here (most of the documentation applies to Darwin as well). A QuickTime/Darwin Streaming Server technology brief in pdf format is available here. Developer documentation is available here. Apple has also posted a good summary of the use and advantages streaming versus http fast-start.
Mailing list support for Darwin Streaming Server and QuickTime Streaming Server is provided informally via the streaming server mailing lists (the users list or the developers list).
Online support resources include the QuickTime section of Apple's support site, and the Streaming Server FAQ.
If you are using Mac OS X Server, QuickTime Streaming Server is already installed. The procedures outlined on this page use the web administration front end for QuickTime Streaming Server. To use web administration you must enable this capability using the Server Admin application in Panther Server. Select the QuickTime Streaming Service and navigate to Access-Settings. Check off "Enable web-based administration", set the password for web administration and save your settings.

If you want to use Darwin Streaming Server, it can be downloaded here. Windows does not include Perl which is required for web administration of Darwin Streaming Server. You can download Active State Perl for Windows here. Make sure to have Perl installed before running the installer on Windows.
Once the Darwin binaries are downloaded, unpack the archive and review the Read Me file, then start the server. On Mac OS X, the server will automatically start after installation. The best way to start the server on Solaris, Linux and Windows is to start up the Perl script that provides web administration:
/usr/local/sbin/streamingadminserver.pl
C:\Perl\bin\perl "C:\Program Files\Darwin Streaming Server\streamingadminserver.pl"
C:\Perl\bin\perl "C:\Program Files\Darwin Streaming Server\WinPasswdAssistant.pl"
http://127.0.0.1:1220
Once you have navigated through the setup screens, you should see the main window for web server administration on your server. The options configured during setup can be changed in the general settings and port settings panes of the administrative web interface.
Note: You can set up the streamingserveradmin.pl to automatically execute when your server is started if you always want to have the server running (i.e. by editing rc.local on Linux). On Windows 2000/2003 Server the installer adds Darwin Streaming Server as a service in the Service Manager.
To test your server, go to another system on your network with QuickTime installed. Open QuickTime Player and select "Open URL..." from the File Menu:

Enter the rtsp url to your server using it's IP address and the filename sample_100kbit.mov (a sample movie installed in the Movies directory on the server):

The player should open up a stream from your server. Compressed and hinted movies can now be placed in the Movies directory and streamed.
This page has some useful information on starting a stopping the server once the server is up and running.