Your scenario data file is the place to reference new creatures, floors, items, and terrains. It would be named somethingdata.txt, where 'something' is the name of the scenario (which you named when creating it).
Make sure to use unused numbers for the objects you put in the data file, or be sure of the consequences. Like Nioca said, reading the docs, especially those on Kelandon's Pink and Pretty Page, is a great way to start. After that, play around with an existing scenario to see how it is constructed. Good luck, and keep asking questions.