Most writers know that an “apostrophe s” is needed to show possession or ownership. Where that apostrophe goes, however, changes depending on whether the word is singular or plural and if it ends in s or not.

If the noun is singular and does not end in s, place the apostrophe between the word’s last letter and the apostrophe s: Jane’s, desk’s, car’s.

Should the singular noun end in s, just place an apostrophe after the word’s last letter. Don’t add an extra s:
CORRECT: dress’ (The dress’ hem needed to be raised.)
WRONG: dress’s

Generally, if the noun is plural and ends in s, place the apostrophe after the s: girls’, cities’, vehicles’. As most plural words end in s, don’t add an extra s:
CORRECT: boys’
WRONG: boys’s

If the noun is plural but doesn’t end in s, add an apostrophe s: men’s, women’s.