A common problem with new users to the Creation Kit is when they load it up and don’t see any assets inside – the most crucial step before you start modding is loading the Skyrim assets.
From Bethesda, look into creating clutter such as wall pieces, raised floors, rubble piles, static clutter, animating objects and more.
Part 2 of the offical Bethseda tutorial on creating a layout in the creation kit, including cave walls, pillars, cliff pieces, dirt, and more.
An official tutorial Bethseda on how to create basic layouts in the Creation Kit, including building a basic room.