Now, I know these are simple rules that most people know. But, a refresher never hurts :) So, here we go.
To show possession:
1. Noun's that do not end in an "s"
In this case, simply add an apostrophe and an "s"
John's car; Stacy's house; Jerry's book; Lupe's lipstick; Frank's wife; Lisa's dress
2. Noun's that do end with an "s"
In general, you'd add an apostrophe and another s
The witness's testimony; the dress's hem; Chris's paper
But when the extra "s" makes the word hard to say, the apostrophe is just added to the end of the word. This has become a matter of preference as well. When it comes to words that end in "s," you can either add the extra "s" or just leave it with an apostrophe.
Mr. Rogers' house; Socrates' essays
3. Plural nouns
With plural nouns that end in an "s" or "es," add the apostrophe to the end of the word
The ladies' room; the teachers' meeting; the parents' appointment; the beaches' tubular waves :D
4. Plural nouns that do not end in an "s" or "es"
Add an apostrophe and an "s"
The children's library; the women's luncheon
5. When you are talking more than one person
This one used to trip me up. If you are showing the possession of something by two people, only the second person gets the apostrophe
Tom and Michelle's house; Jerry and Michael's car; Mike and Laurie's kids
6. Abbreviations and Years
When dealing with abbreviations and years, add an apostrophe and an "s"
the CPA's accounts; the FDA's ruling; the USSR's economics; the USA's treaty; The 1980's; the 1320's
the RNs' graduation; the PhDs' dissertations; the MAs' classes
For contractions, the rule is pretty easy. Just use an apostrophe wherever there are missing letters.
Can not = Can't; Did not = didn't; Should not = shouldn't; madam = ma'am; Hide and seek = Hide 'n' Seek
Its and It's:
Its = possessive form of it. The book has a picture on its cover. The dog has a bug on its nose. The car has mud on its hood.
It's = contraction for "it is" or sometimes "it has." It's raining today. It's cold in the auditorium. I'm sorry you hate the rules, but it's always been this way. ;-D