All Cornelius kegs need to be sanitized before use.
Many sanitizing solutions are available, but my personal choice is Star San. It is easy to use and requires no rinsing saving time with your kegging adventures.
Step one will be to bleed the pressure off the corny keg. This will allow you to remove the cap. To relieve the pressure pull up on the relief valve. If your keg is not equipped with one you can push down on the center of the gas (IN) connector on your Cornelius keg. Either way you relieve the pressure hold a rag over the valve or connector to prevent getting any liquid on you. Sometimes it happens.
After the pressure is gone, pull up on the cap locking handle. Hold the handle @ 90 degrees from the cap and strike it with the palm of your hand or rubber hammer. Once the cap is loose lower it into the keg and twist to allow clearance to remove it from the corny keg.
Add about a gallon of water to the keg and reinstall the cap. Pick the keg up and shake it up a bit. Remove the cap and dump the water out. Do this a couple of times to make sure your keg is totally clean.
Now mix up a couple gallons of your favorite sanitizer in a clean container. Remove the cap and drop into sanitizer. Take about one gallon of the sanitizer and put it in your Cornelius keg. Put the cap back on the keg and pressurize to about 10 psi. or higher until the cap is sealed. This may take up to 30 psi in some cases. Shake the keg up to coat the entire inside of the keg. Install the tap to the keg and let the sanitizer flow out the tap until empty. Remove the Co2 and let the pressure bleed out thru the tap. If you are going to store the corny keg for a while, leave the pressure in it to keep any bacteria from getting in your keg.
Following these steps will ensure your success in successful storage of beer without any problems.