since you do not speak english good, i will try to explain simple.
their is when something belongs to someone.
they're means they are.
there means a place.
let's go through some sentences.
1. (They're there, their) having dinner.
It would be they're because they are having dinner.
2.That is (they're, there, their) dog.
It would be their because they own the dog.
3. Look over (they're, there, their).
It would be there because it is a place.
Try yours on your own, and if you need help comment saying you need help.