I found Empty Keg, 15.5 gallons. what can i do with it. can i get it refilled at a store?

Lots of people seem to think that liquor stores have a tap or something and just pour beer into empty kegs but that is not how it works. An empty keg is called a "shell." Liquor stores trade whatever empties they get back to beer distributors so that the liquor store doesn't have to pay whatever the cost is for those shells. With your empty you can either use it in place of the $20 - $40 deposit for the shell when you get a fresh keg, or you can try to sell it to someone for homebrewing. Or use it as a chair that's fun too.