Use an apostrophe to denote ownership to a singular or plural noun and indefinite pronoun by adding an -'s if the word doesn't end in -s.
Of all forms of punctuation, the apostrophe appears to be in greatest peril of extinction. For proof that the apostrophe should be placed on an endangered species list in some grammarian's office, one needs only to consult the popular press or a sample of student themes. However, because of its ability to denote ownership in a concise way (by avoiding the use of a preposition), the apostrophe plays an important role in the English language. Despite the frequency of its misuse, the apostrophe is a fairly simple form of punctuation to master.