To form a possessive, we add an apostrophe and -s ('s) to a singular noun. When a plural noun ends in -s, we just add an apostrophe (') to the noun. When a plural noun does not end in -s, add an apostrophe and -s ('s) to the noun.
When we talk about something that does not belong to a person or animal, but is part of another thing, we usually use a phrase with of.