Your new 925 sterling silver Magen David is a piece of fine jewelry that will require some gentle care to keep it shining and looking beautiful for years to come.
Sterling silver will tarnish with exposure to air and moisture. Your necklace came packaged in a mini plastic bag, which I recommend using for storage. When kept in a sealed bag, your necklace will tarnish much more slowly.
Your necklace also came with a Sunshine Polishing Cloth, which can be used and reused until it runs out of cleaner (it should last a long time). You do want to be careful with this cloth, since it contains a chemical cleaner. Wash your hands after handling it, and do not rub it on your skin. If you have very sensitive skin, you may wish to wear gloves while using the cloth.
You can use the Sunshine cloth to clean the silver parts of the charm, as well as the jump ring and the chain. Do not rub this cloth directly onto the charm's enamel sections, because the abrasives in the cleaner can damage the enamel. The enamel is perhaps the most sensitive part of this jewelry piece. Enamel does not like direct sunlight, extreme temperatures, or harsh chemicals.
Do not use liquid silver polish to clean the charm.