I don't have time to do this in detail but I hope you pick up enough so you can make a start.

Enable the grid and snapping (make sure you can see the grid). Size of each square should be your basic unit of size (normally the size of your narrowest corridor.

Using the rectangular select tool, and the fill tool set to a nice blue, fill out your walls in blue leaving the rooms and corridors white (using the shortcut 'M' and 'G' keys (Windows) saves a lot of time here).

To get the grid follow this tutorial

best of luck!