Du or Sie?

“You can say you to me” is a popular German joke which you won’t understand unless you speak German.
Now you have already learned we have two translations for the English you: du
(for one person) and ihr (for several persons).
That’s not the whole story. You can only use du and ihr in informal situations, i.e. when you are talking to
• family members
• children
• friends
In all other situations, especially when you are talking to strangers, you will have to use another pronoun, and that is Sie. It’s the same Sie as with sie singen (so 3rd person plural, to be grammatically exact), just written with a capital letter (which no one will hear when you are speaking ‒ obviously). You will also use the
same verb endings as with sie singen (they sing) - (If you speak a language like French or Russian (actually, most languages except English), you will be familiar with this concept. The big difference to those languages is that we do not use the 2nd person plural, but the 3rd person plural in German.).

See the difference?
Beate, du brauchst etwas zu essen.
Herr Meier, Sie brauchen etwas zu essen.

If you read these sentence carefully, you realized something more: We use the du form with first names and the Sie form with Mr/Ms (Herr/Frau) + last names.
There is a logical consequence from this: When someone introduces themselves with their last name, you will use the Sie form. When they introduce themselves with their first name, you can use the du form.
Pretty weird, pretty complicated, and even Germans have trouble choosing the right form from time to time.

If in doubt, use the Sie form. The same applies to a group of persons where you would use Sie if you met someone from the group alone ‒ instead of ihr, use Sie.
Beate und Christian, ihr braucht etwas zu essen.
Herr Maier und Frau Müller, Sie brauchen etwas zu essen.

And here for a few useful phrases:
(jemanden) duzen, du duzt = to say du (to someone)
(jemanden) siezen, du siezt = to say Sie (to someone)
Können wir „du“ sagen? = Can we say du? (offering someone to use du)
Natürlich. Ich bin Martin. (Do state your first name if you’re not sure the other person
knows it.)