It seems to me that Идти is also Indefinite ("что делать?"), so the difference between идти and пойти is in aspect - the former is imperfective, the latter is perfective. Пойти means a one time action in the past, in this case it is similar to verb "сходить", and means a finished action in the past ("Пойти (сходить) в магазин за хлебом" - "To go to the shop for bread"). It can also be used to form future tense of the verb "to go" - "я пойду" or "я собираюсь пойти" or "я хочу пойти".

I have the present, past and future of Ходить and Идти, so how I use the verb Пойти? This is breaking my brain.
If you used present/past/future for the verbs ходить and идти (я хожу / я ходил / я буду ходить or я иду / я шёл / я буду идти) this will mean only the process, like the continuous tense in English (I am walking / I was (have been) walking / I will be walking or I am going / I was (have been) going / I will be going), so you need to use the verb "пойти" for finished action.