Voy is the conjugation of the verb ir for the first person singular in the simple present. The simple present tense is used to talk about habitual actions or routines. This tense is also used to describe things that happen now or in the near future. The verb ir is irregular, so you need to learn by heart conjugations of this verb because there is no any rule that this type of verbs must follows. Finally:
Yo voy a la farmacia luego
