Understand the proper usage of the word you need. The best way to do that is to sound it out. There is a replacement test that will tell you for certain which word will fit. Read your sentence with "you are" in place of the "your/you're," intended. If it makes sense, then "you're" would be right. If not, then "your" would fit. You could also replace "your" with "my" in the sentence. If it fits, then use "your."
Write You're as a contraction or a combination, of the words you and are. Other examples of contractions include doesn't for does not, they're for they are, and can't for can not.
"You're a good friend" means "You are a good friend."
"I don't know what you're talking about" means "I do not know what you are talking about."
Write your as the possessive form of you, referring to something that a person has, something that belongs to the person in question or, the person you are talking to.
"Is your stomach growling?"
"Your book is on the table."
Take a look at some examples. Each of the following examples shows an incorrect use of your/you're, and why it is incorrect.
"I can't read you're handwriting."
Incorrect, because the contraction for "you are" is being used in place of the possessive form of you. I can't read you are handwriting doesn't make sense. It should be replaced with "your".
"If your hungry, then you should probably eat something."
Incorrect because there is no possession in question. this passes the replacement test. Rewriting it with "you are" fits. This should be replaced with you're, or you are.
"I think your very smart."
Again, incorrect. The "very smart" does not belong to the person that you are talking to — this doesn't make any sense. Replace your with you're, or you are.