The possessive of a singular noun is formed by adding ’s (whatever the final consonant).
1. Jane’s book
2. Keats’s poems
3. the boat’s mast
4. Jesus’s teachings
Note that some consider Jesus to be an exception to this rule and insist that its possessive be written Jesus’; other exceptions may include other ancient proper names. It is not necessary to make this exception in your writing. Also note that in the case of Keats’s and others, a spell-check hit indicates only spelling, not grammar.
The possessive of a plural noun is formed by adding only an apostrophe after the final letter if it is an s (or by adding an ’s if the final letter is not an s).
1. the students’ papers
2. the children’s toys
3. the Clemenses’ box seats
The possessive of a pronoun has NO apostrophe.
1. The pen is his.
2. The car is theirs.
3. The decision is yours.
4. The car lost its hubcap.
Note that it’s is a contraction meaning it is. It’s is never a possessive form. It’s a shame it’s such a common error that it warrants its own section.
For more information, refer to The Fourth Deadly Sin: Misuse of the Apostrophe