You're Or You're

A very common mistake so you're not the only one interested in knowing the answer! "Your" and "You're sound the same when spoken but they can never be used interchangeably. Here's why: 

To say, "You're welcome" is the same as "You are welcome!" as there is nothing possessive in the meaning (so we don't use "your")
YOU'RE WELCOME IS THE ANSWER

More on contractions here.