I am no expert, just a native speaker, but there is one general rule of thumb you can use, for starters to figure out which one you need. The best way to figure it out is to ask where it is.
For example, say I am holding a book. It is THIS book. It is in front of me readily apparent. Now let's say I am moving the book about 20 meters in front of me on a table, and going back where Istarted. I point to it. It is THAT book. Not readily apparent. Far away from me, cannot touch it, can barely see it.
Now, for less tangible things, talking about ideas (here is where it gets much harder) You tell a good joke. Your friend says, " THAT'S a good one!" You obviously cannot touch a joke. It is apparent to everybody. But why is THAT used? -Because you told the joke, and you own the joke, so to the speaker it is THAT joke someone over there told. Or how about if you have a gossip with your friends, something shared and apparent to all? You'll say "I like THIS" You cannot touch the gossip either, and it is apparent to everybody, but everybody participates, everybody owns the gossip, not just one guy over in the corner, it is not specific to one person.
By no means is this exhaustive, and there are probably some mistakes, but it is a good place to begin if you want to teach.