Hi Odense,
Welcome to RMXP (The gamemaker & this Forum).
Create a new map, use any of the Interior tilesets. (I'll use 010: Castle Town In)
The very first autotile (the second tile in the first row), is the wall tile.
On layer 1, use the rectangle tool (or flood tool) to fill the entire map with this tile.
The whole map will look black, that's ok.
Now select the tile right below that one. (floor tile).
Draw in a floor to define a room.
Notice that the walls magically appear around the floor. Pretty cool, eh?
Be Well