Here's a simple sample Structured Text ST program. It starts when the PLC application starts and increments a counter variable every second. The CASE statement is used to control the program flow.
Note the PLCcounter variable should be added to the Global Variable List in your program. You can set this as a DINT. With it in the GVL, this will be available to other programs in the application.
For how to import, see this FAQ:
Here's a video it running within PAC IDE software:
Note a PAC320 hardware controller isn't necessary to run this. You can use an application with a CODESYS Control Win V3. Under Online > Simulation. Then you can after Build (F11), click Online > Login. This will create a virtual PLC and allow you to run PLC simulation (Play or F5). Here's a screen capture and project archive: