Use an apostrophe + s (’s) to denote possession: the library’s books, the building’s history. This holds true even if the singluar word ends in “s”: the witness’s testimony, the glass’s contents.
For plural possessives if the plural word ends in “s,” use an apostrophe alone at the end of the word, do not add an “s”: the student’s work (single student); the students’ work (more than one student). If the plural word does not end in “s,” add ’s: women’s rights, children’s dreams.
For proper names ending in “s,” use the apostrophe alone, without adding an “s”: CalArts’ faculty (not CalArts’s faculty); Tom Jones’ microphone, Dickens’ novels.
Never use an apostrophe to pluralize a word. This is a common mistake, especially with names. If you’re inviting the Smith family over for dinner, for example, you’re inviting the Smiths, not the Smith’s.
Do not use an apostrophe to indicate a possessive pronoun: hers, his, theirs, its. “Its” means something belongs to it; “it’s” is a contraction of “it is.”