There are websites that allow you to sell and/or trade guns. You might check with one of them to see what similar guns are selling for. You can also check this website, which provides a value list of various guns: www.usedprice.com/items/guns-and-scopes/index.html.
How much your gun is worth depends on the manufacturer, model and condition of the weapon. If you visit a gun site such as gunsamerica.com and find your gun there you can see what it is currently selling for.
Without knowing the serial number, condition, make and model of the gun, there is no way to estimate the value of the gun you own. Your local gunsmith or a gun appraiser will be able to help you by viewing your gun.