The short answer is that आप is the most formal/respectful, followed by तुम, followed by तू. The longer answer is that it depends on the relationship between the speakers. If you're addressing someone older or in a position of authority (parents, uncles & aunts, teachers, adult strangers, anyone falling under the category of "gentlemen" or "ladies" sort of), use आप. If you're addressing someone younger with whom the relationship is more affectionate (kids, friends, siblings etc), use तू. However, तू can be insulting when used in the wrong context such as when आप is expected. Don't refer to your girlfriend's parent or the college principal as तू, for instance. That would be terrible (unless you actually want to insult them). तुम falls in between the two. You can use it to address the same people you'd use तु with if you want to be a bit more formal or the relationship isn't that close/affectionate yet. Let me end by saying that in India, you tend to assume a relationship when you meet a stranger and refer to them as Uncle/Aunty, भईया (brother)/ भाभी (sister-in-law)/ दीदी (elder sister) etc. Whether you use आप, तुम, or तू sometimes depends on that. A colleague calling you आप is treating you as a colleague, one calling you तू is treating you as a friend or sibling.