Rooms aren't created using BSP trees, they're created using some editor, and stored as a set of polygons in 2D or 3D space. BSP trees are used to store the polygons making those rooms to allow fast and efficient culling to reduce the overhead when rendering. They can also be used to speed up collision detection.
If you're making a simple doom game, and this is your first game, you could just store all your polygons inside a big array. It will be fast enough on modern hardware, even for reasonably detailed scenes. Otherwise, you might want to look up portal-based engines, which probably corresponds more to what you need for a Doom-like game anyways (BSP trees can also be used with portals, but the benefits may not be significant in your case).