Ant must be on your system path (test by typing ant -version at a system prompt)
You must have the environment variables for JAVA_HOME and JAVA_VERSION defined, typically
JAVA_HOME=C:\development\j2sdk1.5.0_07
JAVA_VERSION=1.5
You can check this on windows by typing set JAVA_HOME or on unix by typing echo $JAVA_HOME
FAILURE TO SETUP YOUR ENVIRONMENT VARIABLES WILL PREVENT YOUR FROM BEING ABLE TO BUILD PROPERLY
Now to just build the red5 server and start it running type ant server - This will compile the code and start the red5 server listening on port 1935.
To test the server use either the samples in the "swf" directory or write your own fla to call red5 using rtmp://localhost:1935/oflaDemo as the URI.
To change the default port Red5 listens on edit the conf/red5.properties and change the property rtmp.host_port value to your desired host and port i.e. :
rtmp.host_port = 0.0.0.0:2935
and then restart the server by typing
ant server
Please direct any questions to the red5 mailing list : red5@osflash.org