Brass and Copper will naturally gain a patina over time.  This oxidation is a beautiful aged look, but if you don’t like it you can try a vinegar and lemon scrub or a salt and white vinegar scrub on your copper pieces, or a toothpaste cleaning on your brass jewelry.
You can also try a polishing cloth, my favorite one is the Sunshine cloth.
Some pieces are purposely darkened/oxidized in specific areas to create contrast, bring out texture design and details.  Be careful when polishing your jewelry to not polish those areas.