You can always choose "para". That's because the preposition "a" with the verb "ir" (to go) sounds rather unnatural in spoken-language, even in formal contexts (such as with your co-workers). It is gramatically correct in the appropriate cases, but definitely not necessary at all but when dealing with formal written-language. Anyway, you could, if you want, use "a" when you go to places in which you do something and then when you're finished you leave, such as "mercado" (supermarket), "cinema" (cinema), "teatro" (theater), "banco" (bank), "academia" (gym), "shopping" (shopping). "Ele foi ao mercado", "Eu fui ao banco", "Eu vou ao shopping". But again, in these cases you could use "para" too. The only exception in which I should advise you to use "a", is in: "Eu vou ao banheiro" (I'm going to the toilet). Really the only exception. Otherwise, use "para" and you'll be fine.