1999 Chevy Monte Carlo a/c blowing warm air- replacing seal

To buy the tools needed to vacuum, leak test and charge an a/c system will cost way more than getting it recharged an leak tested at a shop. and if you dodn't know how to do it, you will end up damaging more than you fix. I recommend STONGLY that if you have no knowledge of car a/c systems, leave it alone! That said, you need a good set of manifold gauges, a vacuum pump for vacuuming the moisture out, and a leak detector for obvious reasons.
Your car uses R134a refrigerant, so the low(charging side) and high side use different size connectors so they cant be mixed up. The first thing that must be done is to attach the gauges to the sysem, then attach the pump to the middle(yellow) line, then the left(blue) hose to the low side and the right(red) hose to the high side. open the low side gauge and turn on the pump, the pump will suck out the gases until the gauge goes below zero, the system is now in a vacuum and is removing moisture by boiling it out.
Vaccuum for at least an hour. Now shut off the pump and close the low side. The system should stay below zero, or you have a leak. Your system should have a dye (bright green) in it, use the special glasses to detect the dye on hose ends and all visible parts. find leaks and repair them, then repeat the vacuum proccess until the system can hold a vacuum.
Then attach the recharge hose to the middle (yellow) hose and open the low side and the vacuum in the system will suck out the R134a from the can. When it stops sucking start the car and turn on the a/c, hopefully there is enogh r134a to allow the low pressure cut off to turn the compressor on, continue charging whith can in upright position until it's empty, then close the valve at the can tap and the low side dial., change cans, open the can tap, then press the shreader valve at the low dial until frean is expelled, now open the low side dial and continue charginging, do this until the propper amount of R134a is in the system. It is maked on a lable under the hood. At this point if there are no leaks and the compressor is working your readings on the gauges should be around 30-40 on low(blue) gauge and 150-200 on the high(red) gauge, and it should be cooling in the car vents!