First off, welcome.
Second off, if you've ever been a DM/GM for another system, you already understand improvisation and flexibility. There are a few ways to set up a game:
*Set Exposition- you have a solid exposition, and the party decides what to do and goes at it. Very open, sometimes lacks direction.
*Solid Conclusion- you have an idea how you want a scenario to end, but you don't describe the method of getting there besides a VERY rough framework.
*Set plot- characters go through a series of set events. A little rigid for DFRPG, since the whole system is based on choice (set events happening restricts this).
I personally favor the 2nd option. My most successful games have been quests for an Item, for lack of a better term. I set up the goal, and I establish where the end objective is (but don't tell the players). I let them take whatever path they like to get there. I have some ideas of events/sub-plots, and integrate them as they progress the story forward in whichever direction they like- I just gently nudge the plot/details towards that end. Any other details are the products of their choices/the RNG.
The important thing to take away is that DFRPG is about choices, and their consequences. Also, the system is much less about numbers than others (like DnD), and more about story. If you friends prefer a constant epic-battle game, this may require tweaking somewhat.