You use "it" when you are talking about something that is in the third person, but is not a human. For example, if you are talking about a cat, you wouldn't say "what does this like to do?" Instead you would say "what does it like to do?" Or better said, if you can replace "it" with "she" or "he" then you're using it right. You can't use "this" the same way that you use "he" "she" and "it" unless expressing emotions for something. For example, "I love it" is almost the same as saying "I love this." The only difference is that saying "I love this" implies that you are talking about something close. You can never use "this" when talking about something far.