Sterling silver is an alloyed metal consisting of 92.5% pure silver and the rest other metals. It is prone to tarnishing. The best ways to prevent unwanted tarnish are to wear your jewelry often (the natural oils in your skin help prevent tarnish), protect your jewelry (from chemicals, chlorinated water, lotions, etc), and clean your jewelry- I always include a polishing cloth with every order. I also always recommend storing your jewelry in an airtight container/bag such as a ziplock bag.
Due to the nature of natural turquoise, I do not recommend excessive contact with water. Turquoise will continue to absorb water and that will change the color over time. I recommend removing jewelry before swimming, bathing, and hand washing.