The answer to this can be found in Miller's notes from Act I, Scene 1. They are in the part of the notes that is found before the dialogue actually begins.
In this part of the notes (in the last paragraph of notes before Tituba's first line), we are told that Parris was a merchant before he became a minister. He pursued this profession in Barbados, which is relevant because that is where he met Tituba. He presumably (we are not actually told this) bought Tituba and then (we are told this next part) brought her with him to Salem from Barbados.