1. If a singular noun, add ‘s. Example :
The boy's hands were chapped by the cold.
The dog’s toy was missing.
2. Even if the singular noun is a proper noun (i.e. a name), add ‘s. Example
Richard’s attempts to solve the problem were rewarded.
3. If names ending in s, you can either add an apostrophe + s, or just an apostrophe. The first option is more common. When pronouncing a possessive name, we add the sound /z/ to the end of the name. Example :
Ramesh’s car (or Ramesh’ car)
Gupta’s house (or Gupta’ house)
Geeta’s exam (or Geeta’ exam)