Do not attempt to remove the valve from the propane container. Only a service professional should remove the valve; doing so can be dangerous.
Use up all the gas within the propane container if possible. This makes the disposal process safer.
Talk to your propane suppliers. Sometimes suppliers will dispose of empty containers for you.
Contact a scrap metal yard if your propane container is empty. Some yards can dispose of propane containers.
Use online sites like Earth 911.com to locate more places in your area that might accept your propane container. Type "propane tank" and your zip code in the search boxes.