So let's start with that one.
your = it shows possession
Example: Your belly is huge!
35.5 Weeks (two weeks before delivery)
Sophie and Michael - 3 weeks old
Photo by Alison Claire Photography
To check your usage, just try replacing the word with "you are" to see which one fits.
You are belly is huge!
See, it doesn't work, so you have to use "your."
The opposite is the mistake I see most often, using "your" all the time. If you can replace the word with "you are" then "your" is incorrect. Watch out for that one!