Some inexpensive pre-packaged kits are available, although if you stick with it, you will find your needs will quickly outpace these kits' capabilities. When designing a darkroom, be sure to block all light sources. If there is a window, use black masking tape and heavy gauge aluminum foil over the window, plus blackout shades. For doors, hang a blackout shade in front of the door that hangs to the floor and beyond the door's sides. If this is difficult, try to do most of your work at night, with lights out outside the door to the darkroom.
Ventilation is also important.
An air vent that doesn't let in light is essential, because the chemicals do give off fumes that are not especially healthy if subjected to prolonged exposure. Make sure there is enough space to store everything, including a space to keep prints and extra paper. The best place for chemical storage is under the sink and tray area. A small refrigerator will help you prolong the life of your paper and chemicals, so that too is a good, though optional, investment. Also, a rubberized floor will make it easier on your feet when you're moving around.