for navigation!The first thing to know about a darkroom is what that means - in other words, how dark are we talking about? That depends on the materials involved. Loading film needs to be done in "complete" darkness, since it is so sensitive to light of all wavelengths. Enlarging negatives on paper is generally done with a "safe-light". The type of paper or process determines the type of light required. Typically, deep red lights are needed for lithographic film, but a brighter amber light is acceptable for black and white paper, which is only sensitive to blue light.
An excellent reference for many aspects of analog processing is The Darkroom Handbook by Michael Langford.
There are many levels of sophistication and "elbow room", from commercial large-scale darkrooms all the way down to a closet that barely fits one person. In my case, I converted a spare bathroom, which is convenient because it has a sink, and in my case, there was no skylight or windows to deal with, so a simple rug pushed against the bottom of the door is the only thing I have done to make it light-tight.
The simplest processing is black and white. I started with enlarging negatives which were commercially processed, but have since started to develop the film as well. It takes a bit of equipment, some mandatory, some optional - of which eBay or other sites can be a big help on the budget. A nice enlarger can be $1000 and up, but can sometimes be had for less than $100 used. Of course, you may get something that is incomplete or not working, so "your mileage may vary" as they say.
In terms of black and white chemicals, a steady supply of deionized or distilled water is useful to keep contaminants out, but tap water can be used for some portions of the process quite easily. Generally, film and paper will have different types of developer, but may share other parts of the process, called the "stop bath", "fixer" and "wetting agent". So, for basic developing from start to finish, you may only need five chemicals other than water - two developers, stop, fix and wash. Certain films or papers may have particular requirements for chemicals that may increase your chemical stock, however. Certain processes, such as sepia toning, for example, will require further chemicals.
Most of the standard processes are fairly benign, but they can make a mess and be bad for you and the environment in large quantities. I have refrained from dumping chemicals down the drain, although many people do just that. Sometimes I will wear gloves, but generally it isn't really needed. Some of the chemicals for toning, for example, are extremely toxic and need to be respected. Know what you are dealing with and get SDS (Safety Data Sheets, formerly MSDS (Material SDS)) for each liquid or powder you bring in.
Rather than detailing the entire process, I refer you to Ted's Photographics and Wikipedia.
What I will discuss in some detail are the things that I myself have done: chemicals, films, papers, and techniques.
for navigation!