It's quite easy now days to have an extremely detailed map of a city just seconds after reaching it. In the Middle Ages, when all/most maps were hand drawn and quite scarce (compared to modern standards). How would newcomers have found their ways around?
Word of mouth, mostly.
You have to consider that the notion of going around and visiting multiple big cities throughout one's lifetime is a very modern concept. Tourism as it exists today was most definitely not a part of Middle Age culture.
Basically, if you were part of the upper class you would already have connections within the local community and it wouldn't have been too difficult to get in touch with someone who could meet you when you arrived and then take you wherever you needed to go.
If you weren't well off, you would have had to just explore on your own and ask around in order to get whatever you needed.
In both cases though, you'd probably be visiting the city for a distinct purpose in mind and hence it's not as if you'd get there and then decide on the spot what you wanted to do and where you wanted to go.
And as another poster pointed out, big cities were pretty scarce and people rarely ventured far from their place of birth, so any scenario in which a map or GPS would have been useful would have been exceedingly rare for most people.