Apostrophes

The apostrophe has a ton of rules so I just chose a couple to talk and learn about tonight.

• We use the apostrophe with contractions. The apostrophe is always placed at the spot where the letter(s) has been removed.
  
• Use the apostrophe to show possession. Place the apostrophe before the s to show singular possession.
• Show plural possession, make the noun plural first. then use the apostrophe
• and the only time an apostrophe is used for it's is when it is a contraction for it is or it has
• In plural possessive terms, place the apostrophe after the "s." This will indicate to the reader that more than one person or thing owns the thing possessed. e.g. The students' success was largely attributable to their hard work and dedication.

http://www.grammarbook.com/punctuation/apostro.asp