Northeast Gardening: Garden Box Questions, 1 by LupineGrace

LupineGrace wrote:
I posted this in the Beginner's Landscape forum, but maybe this is a better place. (I live in PA.)

My husband and I are tackling a whole-yard project, complete with a garden box. We've built the frame, but now I have some questions...
Like, how do I fill it? I mean, obviously I'll have to put dirt in it, but do I put large rocks in first? Should I put a barrier between the dirt in the box and the dirt of my yard below? Is regular topsoil okay? I'll be growing flowers mostly, but maybe veggies, too. Can they be grown on opposite sides of the box? Or should they be separated completely?
I also have a ton of questions about which flowers work best together and for my zone etc... but I'll save those for another thread.
Here's a pic of what we have so far. Any helpful tips would be appreciated! Thanks for any help.