While most jewelry these days has a protective coating to delay or stop tarnish, some pieces will, inevitably tarnish a little. Precious metal plated jewelry such as gold, sterling silver, and rose gold will tarnish, as will other metals such as brass and copper. That's just the way of metals. This is not necessarily a bad thing, you know the plating is real right? This issue is very easily taken care of too, so there is no need to stress.
There are at least a couple of methods of cleaning tarnished jewelry, both can be done within a couple of minutes. There are dips and cloths. I prefer the cloths for a quick clean over. These often come with two cloths, one for cleaning, the other for polishing and buffing. Simply run the cloth over the tarnished area a few times and repeat with the polishing cloth.
Dips work quickly, and thoroughly. Simple dip jewelry and buff. Be sure to only dip the metal though, unless your specific product is safe for crystals or acrylic etc.