A: Word that fulfills the function of direct preposition and is used in this case together with a verb of movement "voy", this preposition usually has many uses to express motives, places, prices, but in this particular case it is the function mentioned .
La: It is a singular and female article that is required due to the following word "cafetería" since this noun is also female and singular and allows to give greater coherence and fluency to the sentence.
