Learn German: Du bist da. - You are there.

It can actually mean both! While da often translates to "there", in German it is very commonly used to mean "here" or "present". If a friend finally arrives to meet you, you would say Du bist da to mean "You are here!"

When do I use du for "you"?

Use du when you are talking to exactly one person, and that person is a friend, family member, child, or peer. It is the informal, singular word for "you".

Why do we use bist in this sentence?

Bist is the specific form of the verb sein (to be) that goes with du. Just like you say ich bin (I am) and er ist (he is), you must use du bist to say "you are".

How would I change this into a question, like "Are you there?"

To make a yes/no question in German, you just swap the verb and the subject. So, the statement Du bist da (You are there) becomes the question Bist du da? (Are you there?).