I'm not a native portuguese speaker but i can give you a hand. Since you studied a bit german and spanish maybe it can be helpful to find the difference.
onde : where-> onde está a estação? Where is the station? It used in most of the situation but with aonde is different.
aonde: where to, it's implied a movement so it is used when there is a movement ->Aonde você vai? Where are you going?The sentence above is right because you don't see a movement, so "aonde" is not required.
The same happens with german (Wo-Wohin) and spanish (Dónde-Adónde).