When i turn on ac it blows hot air - 1999 Chevrolet Silverado 1500

Normally due to low freon. Also, notice if the a/c compressor clutch engages, as it will not if the freon is low.

First make sure the compressor clutch is cycling with the a/c on high blow and full cold.
If its not turn engine off wearing eye and hand protection locate one of the high or low side a/c test valve caps, they are mounted into the a/c lines and have plastic screw type caps that screw off to keep dirt etc from entering the test valve core causing it to fail.
Once found and the caps are removed put a screw driver into the valve core to carefully purge the valve core to see if freon is in the system.(similiar to letting the air out of a tire valve)
If no freon comes flying out the system is empty, the system needs to be recharged if found empty, to find a potential leak and or to just simply fix the a/c due to a very slow and hard to detect leak....
the system will need 3 one pound cans of 134a freon(environmentally safe) and a low pressure charging hose/gauge probably total cost approx $70.00 if you do it yourself.
Purchase 1 pound can 134a freon with oil additive, 1 pound freon can with leak dye detector additve and 1 pound can freon with leak sealer additive along with the low side charging system gauge and low side charge hose.
Start the engine, put the a/c on high fan blow full cold find the low side fill plastic dust cap and unscrew it, install the low side charge hose to the oil additive freon can first and install it on the low side, next install the leak dye detector freon, last install the sealer additve freon can, by the time the second pound goes in, the compressor should start cycling and cold air should be flowing from the vents.
Remember the system holds 2 3/4 pounds of freon( or almost 3 cans, each can is one pound) if you over charge the system with too much freon a "blow" valve will let loose causing a loud noise when it releases the needed amount of freon, once the valve relieves the pressure you will be good to go.
You have now charged the a/c system the most economically way possible without having to go to a shop and have the whole system vacumn pumped down and refilled for much more then you have spent by doing it yourself.
If there is a leak the dye will take a few hours to flow through the system and find its way out the leaking seal or connector area, when this happens you will detect a discoloration around the suspected leak area(red, blue or yellow dye)many times a tiny o ring (rubber seal) gets brittle and causes a leak or a line goes bad, repairs to an a/c can be as little as 70.00 and go upwards of $1000.00 depending upon what has defected in the system,if the a/c compressor wont cycle with a full charge(check the low pressure cutout switch first and replace if needed) if still doesn't cycle the compressor and or clutch could need replacement $500.00 easily, the cheapest possibilty is a stretched a/c fan belt to loose to spin the clutch on the a/c compressor therefore not allowing the compressor to get the freon flowing throughout the system to develope cold air flow.