Actually the right use of Du/Sie is a quite tricky thing. Even Germans struggling with it sometimes. In general you can use the Du for all your close relatives like grand parents, parents, siblings, uncles and auntys. As well for your friends and classmates. Sie I would use for older work colleagues or older people you don't know right now. As well for your boss even he/she is younger than you. If you you are dealing with customers you should also use the Sie. To switch from the formal Sie to the informal Du is in general the right of the older person or your boss. If you are not sure about the situation I would stay with the more formal Sie at the beginning.
When you meet sb for the first time, you say "Sie", except when a young person meets another young person, then it's "Du". But the tricky thing is that there is no definition for "young", so there might be two 30-year-olds who say "Du" to each other, but another 20-year old might feel offended if you say "Du" to her/him. In general, if sb is older or in a higher position than you, say "Sie" until they ask you to say "Du". Towards children you always say "Du".
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