The Elements of Style: The Elements of Usage 1 & 2

#1: When you need to form the possessive singular of a noun, simply add 's.

Example: I grabbed Sophie's laptop and put it in her backpack.

There are a few exceptions e.g. Jesus' disciples or Ellis' box of cookies.  However, if you are talking about Achilles' heel for example, it would be better to change it to The heel of Achilles. 

Do not add 's to the end of pronouns.

#2: When you have list of three items of more with one conjunction, you need to add a comma between each. 

Examples: I grabbed Sophie's laptop, charging cord, and book bag. 

Hurley, John, and Stuart are on the basketball team.

Purple, blue, and silver.