Wear your jewelry often. When you're not wearing it, carefully hang necklaces or store jewelry in a hard box to avoid bending and tangling.
You can clean most jewelry with soap and water with a toothbrush, or a jewelry ultra sonic.
To prevent tarnish, use a polishing cloth or a liquid dip (read instructions first if cleaning stones and never dip pearls).
Or you can always send your piece back to be cleaned, repaired or refurbished.