“Your” shows possession. Try replacing “your” with “my” in a sentence. If it works, then use “your”.
Example: “Is this your office?” (This means: Does this office belong to you?)
“You’re” is a contraction or combination of “you are”.
Example: “Please phone me when you’re in Cologne”. (This means: Please phone me when you are in Cologne.)
Don’t worry too much. Even native speakers get this wrong! 😉
