How to use Question Tags?

Hello, and thank you for your question. I can most definitely help you.
If we break down the sentence, we want to look for the pronoun and the verb "to-be" which in this case is "You're" or "You are". When creating a question tag, we always use the opposite form of the verb "to-be". If the verb "to-be" is positive, then in the question tag it will be negative and vice versa. So in your example, the question tag will be "aren't you", and the completed sentence will look like:
"You're coming to the party, aren't you?"

I hope that this explanation helps you to create question tags in the future.