Jane Straus's The Blue Book of Grammar and Punctuation as my reference guide. I'll show you just a few of the basic rules, and I beg you to click on over to her webpage and read the rest. They are very necessary for any writer's toolbox. (Note the correct use of the apostrophe?)
Jane's Rule 1: The apostrophe is used with contractions. For example: She isn't ( She is not), and I'm going (I am going), and you're a writer (you are a writer). Use the apostrophe where the letter(s) is removed.
Rule 2: Use the apostrophe to show possession.
The writer's pen.
The dog's tail.
The superhero's cape.
Place the apostrophe before of the s to show a singular possession. If you have a plural possession and it ends with an s, show it this way (although the extra s is not require, it is preferred and it sounds better) . . .
The Jones's driveway.
Jane's rule #9 is especially important, I think. Never use an apostrophe with possessive pronouns. His, hers, its, theirs, ours, yours, whose. They already show possession.
There are more very important and helpful rules on Jane's webpage. Take five minutes and read them over and your manuscript will rock with perfection.