You should make some floor, wall, and door tiles. In fact, I'd consider doing those before anything else, since those will set the mood/lighting and scale for the rest of them.
I recommend starting with a mockup of a scene whenever possible, instead of drawing assets in isolation. A mockup lets you make sure everything works together as you work on it, and provides context for feedback even before you've finished all the tiles (since the parts you haven't done yet can be roughly blocked in).