2000 Chevy Silverado A/C on & off

My A/C on my truck works good and blows cold air when I get in my truck.After 10-20 minutes it starts blowing warm air.Sometimes it starts blowing cold air again after another 10-20 minutes and sometimes it stays warm.It has been doing this for at least 3 months.It does not matter if it is day or night.I am not familiar with A/C's but the clutch is off when it starts blowing warm air and stays off.What should I do first?Thank You.