What you really should do is learn about the system first.
Freon doesn't expire or disappear. Our AC systems are supposed to be sealed. Much like the freon in your refridgerator (stop looking at it like that...lol, it'* for food cooling). If AC isn't working, there'* a reason and usually it'* because the system doesn't have enough freon. That could mean a huge leak, little leak, but there'* a leak. Once there'* a leak, moisture gets into the system. Moisture is an AC systems enemy.
The proper way to fix an AC system is look for and fix any leaks. Vacuum the system and ensure it holds the vacuum (this proves it is sealed). Then fill it with freon. When doing all of this..there are parts like the orifice tube and drier/accumulator that should be replaced for the best working system. There is also an oil in the system that lubricates the compressor.
00 Regal GS: All kinds of cool stuff.