How do you put freon in a 2000 Oldsmobile alero?

Number one rule with refrigerant is, "If you have to add it, you have a leak." Auto refrigerant is sold in pressurized cans and, to my knowledge, requires certification to purchase and use. There will be a port with a cap on the line coming out of the top of the compressor. Screwing the can on will fill it, but overcharging will reduce the cooling power of the refrigerant.

You can buy it at any auto store. Even Walmart carries it. No need for a cert.