All Sterling silver will tarnish over time, regardless of the price and quality. You can not prevent it but you can slow the tarnish process down.
Store your silver jewellery so that it is not exposed to air. You can store it in airtight bags and also in a air restricted jewellery box. To prevent moisture from the air you can include a piece of chalk to absorb the air.
Keep away from chemicals and exposure: perfumes, lotions, soaps and swimming pool water.
Always add your jewellery last when get dressed.
When you clean your jewellery, wash it in warm water and soap. Dry with a soft cloth to remove water spots and store as discussed.