Many programming language manuals start by showing how easy it is to write a "Hello world" program in that language. In APL we pride ourselves with the shortest ... start APL and type:
APLers are used to the idea of using the session as an input/output interface. The rest of the world might not consider that this qualifies as a program. In mswindows for instance a program might be defined as something we can call from the Run... dialog box or set up as a shortcut on the desktop.
To get our program into a state where we can do that requires a choice between a number of alternative routes. The easiest way is to create an APL workspace which executes 'Hello World' when loaded. Here's an example HelloWorldSession which uses Dyalog APL, but you can do something similar in most implementations of APL.
You may want to do something more sophisticated like writing a GUI application or creating an HTML web page. This can be very easy in APL, but the exact details vary from one APL vendor to another...
Here are a few possible approaches if you are using Dyalog APL:
Here is a TCP/IP communications example using APL+WIN