Learn Italian: Non credo alla magia, ma tu ci credi? - I don't believe in magic, but do you believe in it?

In Italian, the verb credere (when meaning to believe a concept or a fact) is followed by the preposition a. Because you are replacing a phrase that starts with a (alla magia), you must use the pronoun ci. It essentially acts as a shortcut for a + whatever was just mentioned, meaning "in it" or "to it".

Why does the first part use alla if the English translation says "in"?

Prepositions rarely translate word-for-word. While in English we "believe in" magic, in Italian you use the preposition a with this verb. Since magia is feminine, a combines with the word "the" (la) to become alla.

Could I put the ci after the verb and say tu credi ci?

No, tiny replacement words like ci always go immediately before a standard conjugated verb. So it must be ci credi.

Since credi already means "you believe", why is the word tu included?