I find the square marquee selection tool to be very useful. Set it to "add" mode (or hold down shift while using it), and you can simply make selections in the shape of your rooms and corridors. Change to subtract mode occasionally to put walls back in as necessary. Once you've laid out the plan in the selection, save it to a new channel (select > save selection…) You can then find a black-and-white shape in your channels palette. After that, there are several ways you can use the selection. Load it up and stroke it, or use it to make a layer mask and apply layer effects, fill it with a pattern. There are probably a dozen other things you can do.

Take a look in the Tutorials forum for a user named Duvik. My advice is adapted from his Photoshop dungeons tutorial.