Du is used with people that you know very well:
Du trinkst eine Tasse Kaffee.
Du gehst jeden Tag in die Schule.
Sieis the formal form you and it is used with people you don't know very well. It is also used in formal writing and such:
Kommen Sie bitte hierher!
Always remember that its always written in a capital letter and not to be confused with sie which is they.
