Learn Croatian: Sutra moram otići u konzulat po vizu. - I must go to the consulate for the visa tomorrow.

While ići describes the general process of going, otići is perfective and focuses on the departure or the completion of the trip. Since you are making a single, specific trip to fetch a document, otići is the natural choice.

Why do we say po vizu instead of za vizu to mean "for the visa"?

In Croatian, the preposition po with the accusative case is used specifically for fetching or picking something up. You use it when you go somewhere to retrieve a person or an object, like picking up your visa.

Konzulat is in the accusative case. Don't we usually use the locative case for buildings?

You use the locative case for a static location, like when you are already inside the building. Because this sentence involves motion toward a destination, the preposition u requires the accusative case.

How could I connect this sentence to our current lesson about wishes using samo da?

You could express a strong hope about this trip by adding a clause like samo da sve prođe dobro (I just hope everything goes well) or samo da je dobijem (I just hope I get it). Because this is a realistic wish about the future, you use the indicative rather than the conditional.