1. The. You say to the listener that you went to Johnny's house, so it is reasonable that the listener will assume that the door you knock on is the door to Johnny's house. That is an assumed specific which requires the article the.
2. A. The speaker doesn't mention a specific pet, so we can make no assumption of what kind of pet that is. So we must use a, as it is a unspecified pet.
3. The. In the second part of the sentence it becomes clear that they are referring to a specific store which the listener knows about (they have been there together before). Thus the listener can assume which store that is, so the is used.
4. A. There is no preexisting knowledge to the listener (that we're aware of anyway) that there is any specific glass of water that the listener might have. As nothing we can't assume that there is a specific glass of water, A is the most likely answer.
5. A (or the)/none/none. In this case the speaker refers to taking a train from Chicago to New York. It is not clear whether the speaker has a specific train in mind or not. A is the safe answer, but the is also a possibility. City names are inherently specific. Unless there is an expressed or assumable confusion regarding city names (or any names, for that matter) they do not require an article.
Make some mistakes? That's okay. Proper article usage is an advanced part of English. It's not something you can really force learn. Pay attention to it, practice active listening, and get used to assuming and gathering information from the way others used articles. You'll get more used to them as time goes on, so don't sweat making mistakes with them now.