Johns dog was named Spot.
Spot wagged it's tail.
"Its going to be a good day." he said.
"Your a good dog."
John offered some dog biscuit's to Spot.
1.) To show ownership — Spot is John's dog — here you use the apostrophe to show that the dog belongs to John.
Remember: Use an apostrophe to show that a noun is the owner or possessor of something.
Here is where it gets tricky, though: the apostrophe is not used with pronouns. Pronouns have their own words to show ownership — my, your, our, his, her, and its.
Remember: think of the word his when you write its. Both end in s but do not use an apostrophe.
2.) To contract two words into one (that's where they get the word contraction) — Sentence 3 should be It's going to be a good day, he said., because It's is the contracted form of it is. The apostrophe stands in for the deleted letter i in is and forms a bond between the two words.
Remember: if you can separate the word into two words (i.e. it is), then use the apostrophe. If not, then do not use an apostrophe (i.e. it doesn't make sense to say The dog wagged it is tail).
Sentence 4 should be You're a good dog., because You're is the contracted form of you are. The apostrophe stands in for the a in are and bonds the two words together. Since the word your can't be separated into two words, you can't use it in that sentence.
3.) Normally, you should not use apostrophes to form plurals of something. Sentence 5 should be John offered some dog biscuits to Spot. The word biscuits doesn't need an apostrophe because 1.) it is not showing ownership, and 2.) it is not a contraction.
Here's another tricky part (darn those tricksters who made up the language rules!): occasionally, an apostrophe is used to form a plural of a family name that ends in es, like the Jones's and to form the plural of a group of years, like the 90's.