Basic aircond troubleshooting technique:
1. When switch on, compressor got kick in?
2. Ur car got aircond fan? If got, did it kick in after aircond switch on?
3. Check the filter dryer and see if there's a window to check circulating cooling agent. If there is, see if there cooling agent inside, or is there a lot of bubbles? A little bubbles are ok. To much bubbles means need to top up cooling agent. Full of liquid with no bubbles means overfill of cooling agent like what BMW7833 said. Not good to the system.
4. If cooling agent vanish not long after top up, must detect the leak, its either the hoses, aircond radiator, internal cooling coil or the valve.
If leaks, then replace the parts lor, if compressor kong, replace compressor lor, if thermostat fail, then replace thermostat lor, if valve blocked, then replace valve lor, if filter dryer blocked, then replace filter dryer lor.
Dunno what I'm talking about? then go to the workshop and prey hard lor..