How to draw a map you have no idea how to start?

So, I have this world I am working on. It’s a Fantasy World, set in the 1830s-1850s of our timeline with some mild colonialism and such (as colonialism was late). I have all these visions of great cities and docks and such, but no idea what the world itself actually looks like. I was curious if you professional map makers had this problem and how you got passed it.

I’ve made a number of maps, about 5-10 by the time of this post and I am starting to get frustrated. I use Paint.Net, but I am willing to learn “GIMP” if I need to. I just need some advice.