Apostrophes are part of a word to indicate possessive case, contractions, or omitted letters.
The apostrophe always goes after the noun who owns the object.
This is true, even if the noun is plural.
If the noun ends in an 's', it depends on the situation. The easiest rule of thumb is to write what sounds natural to say.
A contraction is a shortened version of spoken and written forms of a words or syllables which is created by omitting certain letters.
Numbers can be shortened by adding an apostrophe in place of the omitted number.
An apostrophe and s are also used to form the plural of letters, numbers, and words referring to words
Time Periods
After Numbers
After Symbols
After Abbreviations
With Possessive Pronouns (its, hers, his, theirs, my, mine, ours, yours, whose)
An easy trick is to remember that the pronoun is possessive, so it wants nothing between itself and the s--aka, no apostrophe!