The correct usage of "Sie" and "Du" is even difficult for us Germans. The first answer here ("Sie" for strangers, "Du" for family members or friends) already gives you a good hint when to use the correct word. You can also ask yourself if you want to call someone by his first name (then typically use "Du") or by his family name (then use "Sie"). There are a lot of exceptions, for example a child that wants to express some respect to an adult person often use "Sie" even if they both know each other well. Or in branches of American companies in Germany the employees call each other with first name and "Du" even if they have never seen each other before. If you have some doubts whitch word is correct, then a good rule is to start with "Sie" and listen what the other person is using in the conversation. In a conversation people often start with "Sie", and then one of the persons (typically the older one) offers his conversation partner the usage of "Du".

Sie = informal you. Use it for strangers and superiors. du = formal you. Use it for anyone you are familiar with including family members!

Sie= förmliche Anrede, für Fremde und höherstehende Personen du=informelle Anrede, für Familienmitglieder und andere vertraute Personen

Sie is also equivalent to the englsih she.