What kind of game is this? Platformer, top-down view, 3D first-person view?
1. Generating a room design randomly may be tricky. Especially if you want these rooms to connect properly to each other, and not just teleport from room to room.
2. There is a basic random() expression you can use. For example: random(100)<30 means a 30% chance.
If you need more RNG features, there's an AdvancedRandom plugin.
3. Use random() or choose() expression. For example:
Set RoomIsDark to choose(0,1)