1. How do I get from Krakotopia to Wizard City? I can go the other way.
When you landed in Krokotopia, you exited from a door inside an underground chamber - Tinu was standing there to greet you. To return to Wizard City use the door again. If you're not sure how to find it, open your Map by pressing M on your keyboard. If you are in the main area of Krokotopia, you will see a Spiral icon indicating where the door is located. This is true of all worlds of the Spiral so you need to pay attention where the door is located.
Alternately, you can always recall to your dorm room by using the little house symbol on your compass. It will return you to your Dorm room in Wizard City unless you have a house equipped.
There's a link at the top of this page that says Player Guide - you will find a lot of valuable information in there, young Wizard, I suggest you spend a few minutes reading it.
2. Is there a death and fire magic trainer in Krakotopia? If so, where?
For quite some time, Falmea will be the one you return to to get more fire spells and fire specific quests. Dworgyn in nightside is your go-to guy for death spells and death specific quests for quite a while too.