Adding 's

's should be added to for the possessive singular of nouns, regardless of the final letter of the noun.

e.g.

James's watch

Charles's kitchen

The teacher's notes

There are exceptions. These are when using ancient proper names ending in es or is. 

Jesus' temple

Moses' laws

Isis' palace

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.

one's calculator

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.