Apostrophes are used in two different ways. The first is to show possession.
Susan’s dog (the dog belonging to Susan)
The organisation’s code of conduct (the code belonging to the organisation)
The boys’ toys (the toys belonging to the boys)
The apostrophe is placed after the whole word that describes the owner or owners. The exception comes when describing something belonging to “it”.
The dog is over there, its feet are dirty. (“its” does not have an apostrophe)
The second use of an apostrophe is to show where letters have been missed out. Shortened words such as these should rarely appear in formal business writing.
isn’t (is not)
can’t (can not)
should’ve (should have)
it’s (it is)
Simple plurals do not have apostrophes
Apples and oranges; in the 1990s; mind your Ps and Qs