Learn Spanish (Latin America): ¿Vas al centro con tu amigo? - Are you going downtown with your friend?

In Spanish, the "you" is built right into the verb. Vas is the "you" (informal) form of the verb ir (to go). Because vas already means "you go" or "you are going", you don't usually need to add the word tú (you), though you could say ¿Tú vas...? for emphasis.

The translation says "Are you going", but doesn't vas mean "you go"?

It means both! Spanish doesn't use a separate word for "are" when talking about actions happening right now. The simple present tense verb vas translates to "you go", "do you go?", and "are you going?". You just rely on the context to know which one sounds best in English.

Why does it say al centro instead of a el centro?

Whenever the preposition a (to) is followed immediately by the masculine word for "the", which is el, they are forced to combine into the single word al. Because centro is masculine (el centro), you must use al. It is actually grammatically incorrect to say a el.

Why is there no accent mark on tu here?

Without an accent mark, tu means "your" (tu amigo = your friend). With an accent mark, tú means the pronoun "you". They sound exactly the same when speaking, but the written accent mark helps tell these two different words apart!