How do I build a foundation for a house?

It sounds so simple, right?  But this issue has always mystified and frustrated me!
I see where it SAYS "Foundations" when you're building a house.  But that appears to be only the coloring/design details ON the foundation, once the foundation is already there.  But how do I put the foundation there in the first place???? *Tears hair out*
I know it has something to do with placing rooms.  But when I use the wall tool to make a room, it's always flush to the ground, NOT with a foundation beneath it.  Ugh, I'm SO frustrated with trying to figure this out.  Please someone help???