@Haru
Welcome to the forums.
A few things to think about:
1. Scale
Decide on a scale - the size everything is relative to everything else. If in the real world, the dimensions of a room are say 10 meters by 10 meters, then in the virtual world, perhaps 1 3d unit = 1 meter. This should work reasonably well for a building, because you likely won't get into sizes in the thousands. So it can be a pretty direct correlation.
2. Map it out
Take a sheet of paper and draw out everything as you want it to be designed. This will save you a lot of headaches later on.
3. Find a good 3D modeling program that you can work with
There are many free 3d Modeling programs out there if you don't have the money, or don't have the inclination, to purchase one.
One of the easiest to get quick room/level design, in my opinion, is Deled 3d. It also has the ability to export to Direct X (which DarkBASIC uses) or 3ds (which DarkBASIC Classic also uses). It also comes with a whole bunch of premade objects such as windows, stairs, chairs, pillars, etc. so you can get to building fairly quickly.
Quote: "and I wanted to know where I would start in the coding. "
Once you get your model made, you can load it into DBC with the LOAD OBJECT command.
The coding from there depends on what your goals are once you have your model loaded into DarkBASIC.
Something to keep in mind:
1. If you want to be able to "walk" on the inside of the model, the normals have to be reversed. There are usually controls in the 3d modeling program that allow you to do this. You can tell when an object has had it's normals reversed when the model looks inside out.
Here is an example of a cube with regular normals. I've placed a red sphere inside which you won't be able to see:
set display mode 800,600,32
sync on
sync rate 60
make object sphere 2,10
color object 2,rgb(255,0,0)
make object cube 1,50
do
turn object left 1,1
sync
loop
And here is one with the normals reversed. You can now see past the outside walls inside of the cube - and you'll be able to see the red sphere:
set display mode 800,600,32
sync on
sync rate 60
make object sphere 2,10
color object 2,rgb(255,0,0)
make object cube 1,-50
do
turn object left 1,1
sync
loop
Notice how you can now see inside of the cube.
Enjoy your day.