First, you need to download IVAC, which is the controller software. Then, you need to download a sector file, which enables you to see a specific airport/airspace. Just look around IVAO for them. The divisions have a special page for them usually. Then, you need to start IVAC, connect, and type in xxx_OBS (xxxx being the airport ICAO code of the airport you're watching. If that callsign is taken, you can use other ways). Then, you can see everything.
If you want to become a controller, I'd say it's best to experiment with the controls of IVAC while as an observer.
If you just want to listen to the pilots, and the ATC, enjoy!
Quite detailed, but so many people get confused with IVAC. Hope this helps!