AC help

I have a 96 XJ with a 4.0L.
My question is why is it my AC squeals like crazy only when I turn it on?
I have just replaced the belt and everything works perfect until you turn on the AC. Also how do you determine what type of refridgerant you use?

Anything made after 1994 I beleive uses 3-134a or whatever its called, not the r12. Buy the stuff with the 3-digit number, lol