What are rules in forming the possessive of singular or plural noun?

Singular possessives are formed by adding an apostrophe s to the end of a noun.

Plural nouns that end in -s, possessives are formed by adding an apostrophe after the existing -s; for irregular plural nouns that don't end with -s, the possessive is formed by adding the apostrophe s the same as a singular noun.

OR

There are two accepted forms for possessive singular nouns ending in s:

• Add an apostrophe (') after the existing s at the end of the Word; for example, "Put it on the boss' desk."
• Add an apostrophe s ('s) after the existing s at the end of the Word; for example, "Put it on the boss's desk."

The -s's is the most commonly used, but if you are a student, use the form that is preferred by your teacher.