First, you only fill on the low pressure side, the cans of freon they sell with the connector and gauge attached will only fit on the low side so it's pretty much fool proof as far as connecting to the right port, like this one
https://www.walmart.com/ip/A-C-Pro-Professional-Formula-Refrigerant-20-oz-California/36158903
It's not uncommon for some cars, more so with some brands like GM, for the system to leak a little over time and need to be topped off. Older GM cars (1998 and older for sure) may leak 12 ounces a year and that is between 1/4 to 1/3 of the total the system holds, that's enough to effect how well it preforms. Both my Jeep and the wife's Buick needs a 12 oz can every year to keep up to good cool air.
First thing I would suggest is start the engine, roll down the front windows and turn the AC on full cold, max blower fan speed and look to see if the compressor clutch is engaged and spinning, if it is then likely it's just low on freon. The clutch won't turn on if the pressure in the system is too low, if it engages then it has enough to work, just not enough to get good and cold. Understand that the clutch will cycle, engage and disengage as the system is running, that normal so you may have to watch it for a few minuets. As the AC runs pressure will climb on the high side of the system, when it gets high enough a high pressure switch will kick the clutch off to keep the pressure from rising too high, then as the pressure drops the clutch will kick back on. It would be easier to have a helper sit in the car and turn the AC on while you watch the compressor clutch, when first turned on the clutch should engage. Below is a video showing a AC clutch kicking on and off, the very front plate will start spinning when the clutch in engaged.
When you go to add some freon you have to watch the gauge on the can and make sure you don't over fill the system, you want to stay in the green or safe zone on the gauge. There will be instructions with the can-hose-gauge combo to follow and there are plenty of youtube videos showing how to do it. It's pretty simple and easy once you've done it a couple times and you can save a lot of money doing it yourself but you do want to make sure you understand the basics and don't overfill, you can always bleed off a little if need be, just don't let the Fed's know your releasing freon into the air LoL.
If your unsure, best to have it checked and serviced at a
shop, if it needs more then 24 oz of freon then it needs to be checked for leaks and you've likely lost some oil from the system and that needs to be replaced along with the freon. There is special oil for the system, it needs a certain amount to keep the compressor lubricated otherwise the compressor will chew it's self up. There again, you can overfill the oil also so make sure your careful.
Last, remember, on a 100 deg day your not going to get 60 deg out of the vents, it can only cool down around 20 deg from what the outside temp is and at idle it won't get that cold, bring the RPM's up to check how cold it get's. To check top performance turn the AC on to blow only to the vents, put a meat thermometer in the vent and see how far it drops with the RPM's up and after a min or so, compare that to the outside temp.
Here's that video on checking the clutch.
