this is a very broad question. you should try and narrow it down. Check to make sure that you have enough refrigerant in your system. That is best left for the pro's. Make sure that you compressor turns on. Locate your AC compressor under the hood. Start your vehicle and have someone turn your AC on. You should see the clutch engaging and the compressor turning. KEEP YOUR ARMS AND HANDS OUT OF THE ENGINE COMPARTMENT AND DO NOT STAND RIGHT BY THE FANBELTS. Also make sure that he fan for the radiator provides enough airflow over the AC compressor. You can check this by running the engine while it is cold and make sure the cooling fans are off. When you turn your A/C on, the high speed fan should come on. The other thing you can check is to make sure that your blower is working properly. When you turn the switch on the inside for your fan, you should feel an increase in output through the vents. The more details you can provide, the closer we can come to giving you a more definite answer. Good Luck