The rule is to put the needed apostrophe before the "s" when a word does not end in "s" already. But if the word does end in "s" or a "z" sound, simply add the apostrophe after the final letter.
Use an apostrophe to show the plural of a number or symbol.
The main problem with apostrophes is that people become confused by seeing them next to an "s" much of the time and decide that whenever a word ends in "s" it is a good idea to stick in an apostrophe, just in case...
Note that the possessive pronouns do not use apostrophes; they are possessive already. So, "its" is the possessive pronoun, while "it's" is the contracted form of "it is."