Its/It’s – This is a really common one, but also one that’s easy to remember. The word “it” doesn’t need an apostrophe to form the possessive.
The dog wanted to get its ball back.
The time at which you’ll want to use an apostrophe is when you’re forming a contraction of “it is.” The apostrophe stands in for the missing letter “i” in “is.”
The dog whimpers to make sure you know it’s upset.
Here’s a sentence using both!
When a dog wags its tail, you know it’s happy.
Whose/Who’s – This is a really similar one. You only use the apostrophe when you’re shortening “who is” into one word.
“Don’t you know whose house this is?” she asked.
“Who’s there?” she asked, hearing a knock on the door.
Your/You’re – Ditto for this one. “You’re” is short for “you are.” Otherwise, just use “your.”
“Your dog is really mean,” he said.
“You’re really mean,” he said.
Their/They’re/There – The difference between “their” and “they’re” is similar to above, but people also sometimes confuse “there” for both of them, in spite of it being a completely different word. Here is each of them in a sentence:
This yard belongs to the Johnsons, it’s their property.
You should knock on the Johnsons’ door to see if they’re home.
I went to the neighbor’s house and Mr. Johnson was there.
Remember: if you’re about to use a contraction, separate it out into the words it’s made up of and see whether those words make sense in the context of the sentence!