The you’re/your error seems to be reaching epidemic proportions, particularly in social media.
As someone once told me (a client), a single grammatical error completely undermines whatever it is that you’re trying to say. (Basically you look like a bit of an arse if you don’t know the difference between 'you're' and 'your'.)
Here’s a very simple guide:
'You're' literally means ‘you are’. The apostrophe is used to signify the missing ‘a’ in ‘are’.
So, if I wanted to say: “You are not very good at English grammar, are you?” I could write: “You’re not very good at English grammar, are you?”
'Your' refers to something that belongs to you.
So something like: “Your grammar isn’t very good, is it?”
If you’re ever confused, just ask yourself: “Do I mean ‘you are’ something, or do I mean something that belongs to you?”
And then try to get it right.
Here are some more examples (none based on personal experience, honestly…):
Your mother asked me if you’re available for lunch on Sunday.
You’re a great guy but I don’t find any of your jokes funny.
When I saw your sexy sister, I wondered whether you’re really twins.
Your computer’s full of inappropriate images. You’re fired.