According to Cornered Cat, the number one rule when approaching a found firearm is to always behave as if it is loaded and armed, regardless of whether it really is or not. This is because many firearms are accidentally discharged when the user believed them to be empty or harmless.
Next, you must insure that your gun will not accidentally discharge in your possession. Even transporting it somewhere could be dangerous, so get in touch with someone experienced with guns. This could be a neighbor, friend, local gun shop owner, or police officer.
After the gun is disarmed and safe to be in your possession, learn more about it. If it is an antique or war relic, you may be able to donate it to a museum or sell if to a collector. If not, you may be able to use it yourself for hunting or self defense.