where do I put the air conditioning fluid in the 2000 Hyundai elantra

I advise to have an ac tech look at it. Because if it's low there is a reason. They will pull a vacuum on system to check for leaks then if no leaks detected they will fill system back up. Ask for dye to be added. ( this way next time it gets low no matter how small the leak you can find and fix it). My guy only charges $50 for this. And if he finds a leak he captures the coolant and all this costs you nothing. Unless you hire him to repair the leak.