Any windows also should be blocked up. If the room is not always going to be a darkroom, you may wish to take a piece of wood cut to fit the window (most windows are recessed into the wall, cut a little bit smaller than this), and then plug the edge holes with rolled plastic sheet. 2 hooks and eyes at the top and one at the bottom should hold it in place.
A countertop or table is a good thing to have. The top should be water resistant at the very least; preferably chemical resistant as well. Old school darkroom design says that you should have a large flat sink to hold developing trays and whatnot, so that when they spill (as they invariably do) it's easier to clean up; My darkroom has 8 feet of countertop, 6 of which alternates between "wet" and "dry" processing space. I find my 6 by 8 foot darkroom more than adequate space for one person; two CAN work inside but it's a little cozy. I usually work in my darkroom alone.o
The enlarger sits on the counter at the far end of the darkroom, with enough working space (about 8 inches) between it and the side wall. The rest of the area is easily converted between film and paper development.
A darkroom seems to collect all sorts of "stuff". A good starting point is to look at a commercial lab and see what they use. You'll definitely need some graduates and pitchers for mixing and measuring chemicals. I personally use a 50 mL graduate and a 300 mL graduate for measuring, some 1L pitchers for mixing and working in, and a large collection of "air evac" bottles. While you don't NEED an air evac bottle for anything other than developer, I find it handy to hold fixer in (stop gets mixed for use and then tossed; I do likewise with most of my developers).
Further, you'll need stirring paddles to adequately mix (plastic tongs do in a pinch), a collection of squeegees (an interesting "pinch" squeegee that gets both sides at once for films, and a big squeegee for prints). I also have a dish drying rack and basin for washing off labware.