When I turn on the a/c I hear a squealing sound from the engine, most likely the compressor. What do I grease or what do I do in general to stop this?

 

Are you sure it's not just the belt slipping?

 

Mopst likely - as soon as your compressor turns on - it causes some strain on the belt and is making the belt slip - clean the valleys of your pullies with the belt off.. and give it another go - DONT use any belt dressing.. that crap jsut makes a mess and is only a temporary fix.

 

Remove the tensioner and spray the he!! out of it with PB Blaster. Not the pulley but the internal spring.They get corroded inside and won't travel far enough to keep tension on the belt.
Or just replace it.