When to use their, there or they're
their
should be used when describing something that belongs to someone
for example: their dog was called Spot
there
should be used when referring to the whereabouts of something or someone
for example: the kettle is over there
they're
should be used before an action (or a verb)
for example: they're going swimming with friends
I before E except after C
If you are unsure whether to put I before E then you can usually follow the rule that I comes before E except after C as long as the e makes the ee sound as in piece, priest, thief.
If the E makes an A or an I sound it's the other way round, as in height, neither, weight.
Watch out for exceptions and if you're unsure