Grammar can confuse a lot of people, partly because it is often horribly taught or even mis-taught. One element that is especially badly taught is the apostrophe, that little comma in the sky that insists on showing up before and after the letter S and in all sorts of other places you didn't particularly want or expect it, bamboozling the innocent writer and often thwarting our ability to communicate clearly. Indeed, the apostrophe is so bewildering to many people that it is probably second only to the semicolon as the most misused punctuation mark.
The two most confusing apostrophe-related mistakes center on two areas: use of the word "it", and use of there, they're and their. We're going to see how well you can do on the latter - but we'll help you out first by reviewing the former. If you mean to say "it is," as in "it is mine," you say "it's." The apostrophe replaces the "i" in "is." If you mean to say "it belongs," as in "its shape is square," then you don't need an apostrophe at all! There is no version where the apostrophe goes after the S.
Now we've cleared that one up, let's move onto the old chestnut of there, they're and their. If you do well, tell your friends on Facebook! If you do badly, perhaps you can tell one friend and appeal to their sympathy, as then they're sure to say, "There, there."