Singular nouns: Add an apostrophe and -s to form the possessive of most singular nouns. (The possessive forms of personal pronouns do not take apostrophes: yours, his, hers, its, ours, theirs.)
The bus's fumes overpowered her.
George Lucas's movies have been wildly popular.
Anyone's guess is as good as mine.
** Pre-1990's, many style manuals allowed simply an apostrophe after a singular noun ending in s:
The bus' fumes overpowered her.
George Lucas' movies have been wildly popular.
As a result, it is typically considered optional to use one or the other construction. HOWEVER, when in doubt, be sure to ask your instructor for his or her preference.
Plural nouns: To form the possessive case of plural nouns not ending in -s, add an apostrophe and -s. For plural nouns ending in -s, add only the apostrophe.
Robert Bly helped to popularize the men's movement.
The clowns/ costumes were bight green and orange.
Compound nouns: For compound nouns, make the last work in the group possessive.
Both her daughters-in-law's birthdays fall in July.
Two or more nouns: To signal individual possession by two or more owners, make each noun possessive.
Great differences exist between Angela Bassett's and Oprah Winfrey's films.
Basset and Winfrey appeared in different films.
To signal joint possession, make only the last noun possessive.
MacNeil and Lehrer's television program focused on current issues.
MacNeil and Leher participated in the same program.