The first thing to keep in mind when trying to decide whether or not you will need an apostrophe ('), is that the apostrophe takes the place of another letter (such as with contractions), except when used to indicate possession of an object or idea. So, for example:
"are not" becomes "aren't"
"It is" becomes "it's"
When creating a possessive, the apostrophe is used to indicate ownership:
"the cat's meow"
"the bee's knees"
"the rat's donkey"
Now, when using a pronoun, no apostrophe is needed, because the pronouns all have possessive versions:
When it comes to plurals, many people become insecure about apostrophe usage. To simplify this, consider the following words:
Not all of these are plural, but they all end in "s". This is where the confusion lies. The important thing to remember here is that to make these into possessives, singular words will take an ('s), while plural words only take (').
So, the list above would look like this:
"Mrs Jones's car"
"The Joneses' pool"
Keep in mind that singular words only take ('s), not ('), NO MATTER WHAT LETTER OR SYMBOL THEY END WITH. Take these examples:
"My pass's lob"
Hopefully this helps to clarify things!