92 sts A/C question

I just got a low a/c refrigerant message on my dash. I try to do all my own repairs, but ive never done anything with the air conditioning. the only thing i could find at the store was R134a, but I don't think that's the right one. Any advice is appriciated.

Also, how can you tell which is the low pressure port to add more refrigerant?

Your car probably takes R-12, since it is older. You should find out exactly. It should say under the hood on a sticker somewhere. The sticker is usually yellow. If it takes R-12, only certified AC personnel can get this, and it costs alot.. about $35/lb. If you have an R-12 system, you may be able to find the leak by just performing a visual inspection at the AC components for signs of compressor oil.

Low side will have a smaller fitting than your high side.