The most important rule about the word "this" is not to use it all by itself. "This" refers to particular things and should be used with a noun unless you are using it in dialogue. For example, you would say "this couch," "this carpet" or "this lawn" to point out which one of several possible couches, carpets or lawns you are talking about.
If you are not trying to show the reader which one out of several things you are talking about, you need to use something other than "this." For example, instead of saying, "I'm going to tell you about this." You should say, "I'm going to discuss the problem of global warming." In other words, the best way to get rid of "this" is to replace it with the specific thing you are referring to in the first place.