Here are the rules:
If the noun is singular and doesn't end in an "s," add an apostrophe and an "s."
Example: The car's horn was loud.
If the noun is singular and ends in an "s," add the apostrophe after the "s."
Example: The species' features showed how it can survive in water.
If the noun is plural, add the apostrophe after the "s."
Example: The cats' beds were in the back of the house.
Its vs. It's
Its is a possessive pronoun.
Example: My neighbor's dog didn't like to sleep in its dog house.
It's is a contraction of "it is" or "it has."
Example: It's fun when Grandma comes to visit.
Their vs. There
Their is an adjective that means something belongs to someone.
Example: It was their house.
There is an adverb that means at that place.
Example: Place the box there.
Your vs. You're
Your is an adjective that shows something belongs to you.
Example: Your phone has been ringing all day.
You're is a contraction of "you" and "are."
Example: You're going to love your car.
For more examples of frequently misused words, check out the YourDictionary article "Commonly Confused Words."
Basically the rule is that you use "I" if you are the subject of the sentence and you use "me" if you are the object of the sentence.
I is a subject pronoun and refers to the person doing to action of the verb.
Example: I went to the store.
Example: Tom and I are friends with the teacher.
Me is an object pronoun and is the person to which the action is done.
Example: My mother gave me twenty dollars when I went to the store.
Example: The teacher should ask Tom or me.
Still confused? Here's a tip: To decide whether to use "me" or "I" with another name, just remove the other name.
For example: If you remove "Mary" from "Mary and I went to the park." you would be left with "I went to the park." That sounds correct because it is correct.
However, if you remove "Mary" from "She gave the book to Mary and I." you would be left with "She gave the book to I." That sounds incorrect because it is incorrect.
Example with preposition at the end: What street does she live on?
Correction to change the position of the preposition: On what street does she live?
Example: She parked between the two cars at the curb.
Among is a preposition that refers to things that are part of a group.
Example: She placed her gift among all the gifts on the table.
For more grammar tips, rules and information check out the YourDictionary "Grammar" section.