Others have given good and (mostly) consistent advice, and I concur with the overall themes. My own advice is this:
- Go all-manual. You can always upgrade to a camera with as much automation as you like later. If you buy used, the prices are low enough that the camera purchase price is pretty minor; you'll probably pay more for 2-5 rolls of film and processing than for a basic used camera. A manual camera has the advantage of not tempting you to use automation. This will force you to slow down and think about what you're doing, which will help you learn about things like motion blur, depth of field, backlighting, etc.
- Buy used. Consider your first SLR camera a try-out. Don't buy a new camera with all the features you think you might want; instead, get an inexpensive used one so you can learn the basics and figure out which features you'll really want for the long term. After a few months, buy a new or used camera with those features. This will then give you two cameras, which can be handy. You can load them with two different types of film or use your first (presumably less valuable) camera in environments that might be hazardous for the camera.
- Choose your mount. Most SLR manufacturers use lens mounts unique (or almost unique) to the manufacturer. For instance, you can't fit a Canon lens on a Minolta camera. There are a few exceptions, though. M42 screw-mount (aka Pentax screw mount or Praktica screw mount) lenses fit many makes from the 1970s and earlier, and a few later models. The Fujica ST-801 mentioned by magic823 is an M42 camera. (This was my first SLR, and it makes a good learning camera.) M42 lenses take longer to change than do bayonet mount lenses, though. Another multi-make mount is the Pentax K-mount system, which was used by many manufacturers, including Pentax, Ricoh, Chinon, Cosina, and a few others. A few of these are still making cameras. The most popular mounts are probably M42, K-mount, Canon, Minolta, and Nikon (in no particular order). I'd stick to one of those types if practical -- but if you don't invest in extra lenses at first, you can choose to switch mount types with your second camera and not throw away a big investment. Note that many manufacturers have made cameras in multiple mounts -- particularly M42 and a more recent bayonet mount. (Everything except M42 and a few rare types are bayonet mounts.)
FWIW, I'm most familiar with K-mount cameras. If you're interested in them, you might want to consult this table, which summarizes most of the K-mount cameras made between 1975 and 1994. Try loading the table into a spreadsheet or something and sort by the features that you want to have (or not have). You can then search for those models on eBay or take a list to a local used camera shop. Perhaps somebody else can suggest similar resources for other types of cameras.