There is a difference between "Du" and "Sie".
You can use "du" in your family and when you address friends or children - or if you agreed with the other to use "Du". Additionally you can use it in environments where it is used as a kind of group language.
In almost all other cases you have to use "Sie". This is also called "Höflichkeitsform".
The difference existed similar in the English language, too, but in the modern English it is not available anymore.
So it depends on your context whether to use "Du" or "Sie".
If you are a teacher and speak to adults, use "Sie", if you speak to children (up to 14 years old) use "Du".
If you speak to a group of children, use the plural of "du" - this is "ihr": "Ihr habt das falsch ausgesprochen!".