First, you need a copy of Descent itself. If you have your old installation on a hard drive somewhere, that will work, even if it doesn't still run. What you really need are the .hog and .pig files. If you do not still have a copy of the original game, you can try picking one up on Ebay or something.
Next, you need the source port most pilots use: Rebirth with Retro Mod.
If you are running Windows, use the Rebirth installer. Make sure Retro Mod is being installed, for both D1 and D2 if you have both, and make sure to get the Rangers Anarchy and DCL mission packs.
If you are running on Linux, download the source for the latest Retro Mod and compile it for your system.
You should now be able to launch Descent and fly around a bit! Take a moment to make sure everything works before moving on to the next bit.
Mouse is a much more viable option than it used to be. About half of top pilots these days are mousers. If you wish to go this route, it is recommended that you get a good mouse, and a keyboard with good anti-ghosting. The Microsoft Sidewinder X4 is a relatively inexpensive gaming keyboard that is used by several top Descent pilots.
You will probably find it is worth it to get a gaming mouse, for the accurate sensor and improved ergonomics. The Roccat Lua is an inexpensive option that performs well, and which sees competitive use in Descent.
The Microsoft Sidewinder 3d Pro is still widely considered the best joystick for Descent. However, since it requires a gameport, it is not easy to use at the moment. If you are a returning 3d Pro pilot, the easiest/cheapest way to get back into the game is to use its modern USB equivalent: the Logitech Extreme 3d Pro.
The Logitech stick does everything the old Sidewinder did -- it has a hat and a twist. The action is different, though, and has been described as "stiffer" and "wider" and "like driving a big truck". It is a right-handed joystick, not an ambidextrous one. On the whole, the Logitech stick is considered inferior to the old Sidewinder, but it is an inexpensive, fully functional alternative that works right out of the box. Notably, LoNi_ used it for a long time, and was able to be competitive at the gold level with it . . . but did eventually go back to the 3d Pro. :)
If you want use the old 3d Pro instead (and many pilots do), you will first need one. You can typically buy them used on Amazon or Ebay for about $20. It is also worthwhile to check Craigslist and even your local thrift stores. If you find yourself needing to do maintenance on an old one, this guide may be helpful.
To use the 3dpro on a modern system -- through a USB port -- you will need an adapter. Do NOT get a generic gameport to USB adapter! We have not found an off-the-shelf adapter that gives the full functionality of the stick; you lose either the twist or simultaneous button presses. (If you do find one that works, please let me know so I can update this guide.)
Get an adapter designed for the 3dpro, and preferably one designed for Descent. Grendel's adapter is the gold standard. You can find the original thread about it on the DBB. That thread provides all the instructions you need to build your own, provided you know how to solder. Alternatively, pilots in the community occasionally produce batches of them. If you need one made for you, you can ask around on Facebook or in the DBB thread.
Right now, your best bet to get a 3d Pro adapter is to talk to Arch_Angel / Adam Gensler on Facebook. There isn't a run going at the moment, but you may be able to persuade him to build you one if you're very nice. At cost, they are a little over $60 shipped.
It is also possible to use the 3dpro without an adapter, by building a system with a gameport and running Windows XP. This is a viable (if expensive) approach, but there are several pilots who have done it, and it is known to work. The version of Descent commonly played now does run just fine on XP.
Do make sure to calibrate your joystick in Windows, and to set the sensitivity in Descent itself. If you have trouble with your ship constantly spinning, increase the size of the dead zone in Descent (options/controls/sensitivity & dead zone).
There are two convenient level packs maintained for you. You should get the Ranger level packs for D1 and D2 if you plan on playing in anarchy. You should get the DCL level packs if you plan on playing ladder matches.
If you are planning to play on the ladder, take a look at the core levels: Vamped, Logic, Athena, Black Rose, Nysa, Fuzed, Neptune, Take2, Ascend, and Lurk. Most games happen in one of these levels, and you can be challenged to play any of them. If you don't know them already (and Logic, Take2, Ascend, and Lurk are modern additions), you should make getting to know them a priority!
The Descent community primarily hangs out on the Descent Rangers Mumble Server. On the typical evening, you can find about a dozen pilots there, looking for all types of games -- Ranger games, fun games, ladder matches, or even just hanging out.
To join them, you will need a copy of Mumble. Get that running, and join the Rangers server. The server is run by the Descent Rangers, but Descent players of any type are welcome. Do remember that it is the Rangers' server, and respect their rules.
You can also meet pilots and arrange matches through Facebook. There are Facebook groups for DCL and Rangers, as well as several groups of Descent pilots. The Old School Kali/Kahn Group is the big one.
It is also still possible to meet pilots on Kali, but Kali is no longer the Descent community's primary hangout. If you want to check it out anyway, but have lost your Kali login, there is a Web Interface you can use.
The typical expectation is to arrange a game over mumble (or sometimes Facebook) before launching Descent. Ask around to see if anyone wants to play, but be quiet at first when entering game channels; it is possible someone is already in a fairly intense game.
It is possible to simply launch a game and wait for people to join (or watch for one to appear on the tracker) like we did in the old days . . . but be prepared to wait a long time -- possibly hours. This is not recommended for new pilots, as you will probably want help getting set up before getting shot at.
You can see who is currently playing using Arch's Tracker. The archive will also show you how many games have happened in the last month, who played, and what the scores were. It also tweets when a game is starting; follow @RetroTracker. Cool, huh? Thank Arch.
If this is your first time playing, ask an established pilot to host the game. Kali is no longer needed -- Retro has its own tracker, and will see hosted games automatically. Just go to the Multiplayer menu, "Find Lan/Online Games", and it should be there. Make sure you are running the same version (should be Retro 1.3) as the other pilot. If the game complains about a "protocol mismatch" when you try to join, you picked the wrong one.
If you don't see the game, it's possible that you have Descent configured to use the wrong tracker. There are two right now -- talk to the pilot who hosted the game. By default, Descent uses the Rebirth tracker. To switch it to the (more popular) Retro tracker, follow the instructions here.
If you connect to your home network over wifi, you will probably find that the connection is good enough for casual Descent, but not for competitive play. Wifi typically introduces variable lag and some loss. As a bad connection is considered an advantage in a ladder match, you may find that some pilots refuse to play you until you are wired. While a wifi connection is good enough for getting back into the game, most pilots upgrade to wired after a few games.
Rebirth comes with the ability to host a game out of the box, but if you connect to the internet through a router (which most people do), your hosted games won't be visible outside your home network. The technically simplest way to solve that problem is . . . not to connect through a router. Try networking your computer directly to your cable modem (or whatever the last stop is before your ISP). If there is only one computer in your house, it is possible you do this already; try hosting a game and see if it works.
To verify that your hosting is working, in Descent, go to "Multiplayer / Host Game". Get it all set up and started, and then alt-tab out of Descent and go to the tracker. If you see your game, hosting works. If you don't, and especially if Descent complains "No Response From Tracker", it doesn't work.
You can host a game through a router, but this will require you to set up port forwarding for UDP traffic on port 42424. You can google port forwarding to find a number of guides for getting it done. Briefly, here is how to do it:
To log into your router, you first need its IP address. On Windows, run "ipconfig" from a console and look for your default gateway. It should be an IP address starting with 192.168.x.x or 10.0.x.x. Put that address into a web browser, and log in. It should ask you for a username and password; if you don't know it, consult the manual that came with your router or google the router model. You may need to do a hard reset to reset the password to the default.
Once you are into your router, you need to set up incoming UDP traffic on port 42424 to go to port 42424 at your computer's IP address. Pretty much every router has a way to do this, but they're all different. Look for a section labelled "Port Forwarding" or "Virtual Server", usually in some sort of "Networking" section. It's also pretty useful to set up your computer with a static IP, while you're in there -- otherwise you'll have to do this every time there's a power outage.
Once you have port forwarding set up on your router, try hosting a game. It should work, but it almost never does the first time. Check your router settings and try again. ;)
If those steps are technically intimidating, or you tried them and they didn't work, or your home network is more complex than that, get help on Mumble. There are always pilots willing to help you set up hosting.
If you connect to the internet through a router you don't control, you are out of luck. You can only play in games other people host. Sorry. We have our best people working on that. Your best bet for ladder matches is to ask the other pilot to host (most people can), and in a dire emergency, get someone with a good connection to you to host for you.