Create an area and choose "PlayerStart" in the Area dropdown for the starting position, the blue line indicates which way you're facing when you start.
To test the level simply start SomaDev.bat in your soma folder, if you don't have it you can simply create it by making a .txt file and saving it as SomaDev.bat with the following content:
Soma.exe -user Dev -cfg config/main_init_dev.cfg
Start the .bat and press F1 once in-game and click Load Level on the bottom right.
Edit
Ironically I know that but I have no idea how to make a proper light connected to a switch or a proper roof and wall inside a room(Beause walls only have textures on one side?). I wish I knew how so I can make some custom stuff myself.. hopefully there will be basic tutorials for these things.