The reason of silver oxidation is sulfur. It is contained in various chemicals, eggs, air etc. But it does not imply that your ring will definitely turn black. In usual circumstances it will happen only after 50 years.
Then why for some people this process goes faster?
One frequent reason for that is sweat, which, apart from salines, contains amino acids, which in turn contain sulfur. The contact with sulfur-containing chemicals oxidates silver metal. If you have extra sweat production or exercise a lot, it is likely that your silver jewelry will blacken faster. In the first case we suggest rhodium plating (specify it in the comment to order), in the second case we suggest to take jewelry off when exercising.
Here we also answer the common question: "Usually my silver rings, brought in jewelry stores, do not turn black, why?". This is because silver jewels from stores are always(!) plated with rhodium, which is an non-oxidizable platinoid Also we suggest to take off rings when working with chemicals, because they can unpredictably affect your ring if contacted with.
There are plenty of instruction in the internet on how to shine silver, but if you are afraid to do it by yourself — you can bring it to us or any other jewelers.