Before you can install Linux, you'll need to be sure your machine is Linux-capable, and choose a Linux to install. The Linux Pre-installation checklist may help you organize configuration data before you begin.
What kind of system is needed to run Linux? This is a good question; the actual hardware requirements for the system change periodically. The Linux Hardware-HOWTO, http://www.linuxdoc.org/HOWTO/Hardware-HOWTO.html, gives a (more or less) complete listing of hardware supported by Linux. The Linux INFO-SHEET, http://www.linuxdoc.org/HOWTO/INFO-SHEET.html, provides another list.
For the Intel versions, a hardware configuration that looks like the following is required:
Any 80386, 80486, Pentium or Pentium II processor will do. Non-Intel clones of the 80386 and up will generally work. You do not need a math coprocessor, although it is nice to have one.
The ISA, EISA, VESA Local Bus and PCI bus architectures are supported. The MCA bus architecture (found on IBM PS/2 machines) has been minimally supported since the 2.1.x kernels, but may not be ready for prime time yet.
You need at least 4 megabytes of memory in your machine. Technically, Linux will run with only 2 megs, but most installations and software require 4. The more memory you have, the happier you'll be. I suggest 8 or 16 megabytes if you're planning to use X-Windows.
Of course, you'll need a hard drive and an AT-standard drive controller. All MFM, RLL, and IDE drives and controllers should work. Many SCSI drives and adaptors are supported as well; the Linux SCSI-HOWTO contains more information on SCSI. If you are assembling a system from scratch to run Linux, the small additional cost of SCSI is well worth it for the extra performance and reliability it brings.
You will need a 3.5" floppy drive. While 5.25" floppies are supported under Linux, they are little-enough used that you should not count on disk images necessarily fitting on them. (A stripped-down Linux can actually run on a single floppy, but that's only useful for installation and certain troubleshooting tasks.)
You also need an MDA, Hercules, CGA, EGA, VGA, or Super VGA video card and monitor. In general, if your video card and monitor work under MS-DOS then it should work under Linux. However, if you wish to run the X window system, there are other restrictions on the supported video hardware. The Linux XFree86-HOWTO, http://www.linuxdoc.org/HOWTO/XFree86-HOWTO.html, contains more information about running X and its requirements.
You'll want a CD-ROM drive. If it's ATAPI, SCSI, or true IDE you should have no problem making it work (but watch for cheap drives advertising "IDE" interfaces that aren't true IDE). If your CD-ROM uses a proprietary interface card, it's possible the installation kernel you're going to boot from floppy won't be able to see it -- and an inaccessible CD-ROM is a installation show-stopper. Also, CD-ROMs that attach to your parallel port won't work at all. If you're in doubt, consult the Linux CD-ROM HOWTO, http://www.linuxdoc.org/HOWTO/CDROM-HOWTO.html for a list and details of supported hardware.
So-called "Plug'n'Play" jumperless cards can be a problem. Support for these is under active development, but not there yet in the 2.0.25 kernel. Fortunately this is only likely to be a problem with sound or Ethernet cards.
If you're running on a box that uses one of the Motorola 68K processors (including Amiga, Atari, or VMEbus machines), see the Linux/m68k FAQ at http://www.linux-m68k.org/pub/faq/faq.html for information on minimum requirements and the state of the port. The FAQ now says m68k Linux is as stable and usable as the Intel versions.
You'll need free space for Linux on your hard drive. The amount of space needed depends on how much software you plan to install. Most installations require somewhere in the ballpark of 200 to 500 megs. This includes space for the software, swap space (used as virtual RAM on your machine), and free space for users, and so on.
It's conceivable that you could run a minimal Linux system in 80 megs or less (this used to be common when Linux distributions were smaller), and it's conceivable that you could use well over 500 megs or more for all of your Linux software. The amount varies greatly depending on the amount of software you install and how much space you require. More about this later.
Linux will co-exist with other operating systems, such as MS-DOS, Microsoft Windows, or OS/2, on your hard drive. (In fact you can even access MS-DOS files and run some MS-DOS programs from Linux.) In other words, when partitioning your drive for Linux, MS-DOS or OS/2 live on their own partitions, and Linux exists on its own. We'll go into more detail about such ``dual-boot'' systems later.
You do NOT need to be running MS-DOS, OS/2, or any other operating system to use Linux. Linux is a completely different, stand-alone operating system and does not rely on other OSs for installation and use.
In all, the minimal setup for Linux is not much more than is required for most MS-DOS or Windows 3.1 systems sold today (and it's a good deal less than the minimum for Windows 95!). If you have a 386 or 486 with at least 4 megs of RAM, then you'll be happy running Linux. Linux does not require huge amounts of diskspace, memory, or processor speed. Matt Welsh, the originator of this HOWTO, used to run Linux on a 386/16 MHz (the slowest machine you can get) with 4 megs of RAM, and was quite happy. The more you want to do, the more memory (and faster processor) you'll need. In our experience a 486 with 16 megabytes of RAM running Linux outdoes several models of expensive workstations.