So I am building a house right now, or trying to, and I've made the floor, walls, and doorway. The only trouble I seem to be having so far is how they all fit together. Is there a way to make these snap together?
You can activate the Snap option to make the vertices of your walls stick together, it will also work in Edit mode. As Yash said, you could also use a Boolean modifier to merge some walls, but it won't always give good result. But as AzulShiva said, maybe the best is to model your walls within only one object, it will simplify your work, unless you have a very particular reason to separate your walls.
I agree with Moonboots, and would just add that if you're using Snap in Edit Mode, then having Snap set to 'Vertex' and 'Active', selecting all the vertices you want to transform, with your desired source vertex selected last to make it active, and then hovering over the target, yields the most predictable results. I almost never have my Snap set to anything else.