usually, we use "it's" when we are answering a question like "what is this?" We also use it when we are talking about something that is known or easily understood. Example: "It's cold outside." "It's a good thing you came early." We use "this is" when we are demonstrating or explaining. Example: "This is a rare coin that I got on my trip to Germany." "This is a great plugin that every website owner should use." There are more cases for using "this is" or "it's," but these explanations will give you a general idea.