Brass darkens with wear, depending on body chemistry. If you don't like the dark patina, try to avoid getting your brass jewellery wet. And put them back in your jewellery box/pouch when you are not wearing them.
Jewellery Cloth is a good option to remove the tarnish. Here are some recommendations from me: stainless steel jewellery cloth, The chrome shine polishing cloth from Blitz. For a more affordable option, you can also use fine Steel wool (#00,#000, #0000)
A commercial brass cleaner, like "Brasso", will restore your piece to its original shine. If they are unavailable, use a soft toothbrush and toothpaste to polish or soak the piece in coca-cola for a few minutes.
Please avoid any gemstones or painted area using the methods above
Gold Plated JewellerySome of my chains, ear hooks/hoops/wires, pins and jump rings I use are 14k/18k gold plated, even though they are gold plated, they do go tarnish eventually when the gold wears off. Substances like oil, nail polish, nail polish remover, chlorine, perfume and sweat react with metal/plated jewellery and will cause it to tarnish easily.
You can clean your plated jewellery with a cotton ball or a very soft cloth to remove any dust and dirt it has acquired. Gently rubbing the surface of your gold plated jewellery using a soft jewellery cloth also helps restore shine. If your jewellery needs more cleaning you may clean it with warm, soapy water with a soft brush.
The chrome shine polishing cloth from Blitz is another good option to restore shine with gold plated jewellery, but note that it does not restore the gold.