Sterling silver, brass, and copper will all tarnish over time. They’ll either naturally oxidize just from being out in the open (oxidization is exposure to air) or they will react with moisture and skin oils to tarnish. You can slow both of these processes by storing your jewelry in an air-tight or anti-tarnish environment, and cleaning it thoroughly after each wear.
If your jewelry does tarnish or oxidize, you can easily polish it with a jewelry polishing cloth, like our Sunshine Cloths, available here.