's should be added to for the possessive singular of nouns, regardless of the final letter of the noun.
The teacher's notes
There are exceptions. These are when using ancient proper names ending in es or is.
Often, these are written as
The temple of Jesus
The laws of Moses
The palace of Isis
The pronominal possessives - hers, its, theirs, yours and ours, don't have an apostrophe. Indefinite pronouns do if they are showing possession.
somebody else's cookie
It's, means it is. Its means, it belongs to.
It's unwise to poke a bear when it's eating its food.