Step 1: Gather Materials
1. A computer (seems obvious but still...) which needs an OS.
a. Knowledge about your computer's hardware (see points 3 and 4 below).
b. A bootable CD or DVD drive, or bootable USB port.
2. A boot CD or DVD (or bootable USB thumbstick) of your chosen OS.
3. Drivers for all your hardware. This is especially critical for RAID arrays; if your OS does not "see" the RAID array without a driver, you can't install the OS on the RAID array.
4. Drivers for *ALL* your hardware. I can't emphasize this enough. In order of importance, you *MUST* have drivers for:
a. The motherboard chipset, if there is a special driver for it.
b. RAID card or other drive controller card. Some motherboards will have this built-in.
c. Network adapter.
d. Sound card.
e. Video card.
5. A good anti-virus package (yes, even for Linux and Mac). I personally recommend http://avast.com/eng/programs.htmlAvast or http://free.avg.com/download-avg-anti-virus-free-editionAVG for Windows users. Both are free for non-commercial use. Avast also makes Linux and Mac editions.
6. A suitable archiver. for Windows, you should consider 7-Zip or AlZip (and RARzilla for later). Linux and OSX will include gzip. You'll want unrar and unzip later, but they aren't critical now.
Point 1a is the most critical here; failure to identify your hardware will definitely cause issues, ranging from some things not performing well, to complete failure to install the OS, leaving you with a very expensive doorstop.
Drivers, anti-virus, and archiver(s) should be burned to a disk or disks also (and if they came in an archive package, such as ZIP files, you need to unzip them on the disk(s)).
Step 2: Boot Your New OS
Either set up your BIOS to boot from CD/DVD or USB, or use the boot menu (if you have one) to select CD/DVD or USB as your boot device.
Step 3: Install RAID Driver
Step 4: Format and Partition
Note that the precise method for doing this will vary with the OS you chose; however, the text on the screen will tell you what the installer is doing, and ask you for your decision(s) regarding any options. Read the screen!
Seagate provides extensive details on installing OSX: http://www.seagate.com/ww/v/index.jsp?locale=en-US&name=sata-mac-install&vgnextoid=050ae579ea085110VgnVCM100000f5ee0a0aRCRD
Step 5: Sit Back and Relax
Step 6: Who Are You?
Step 7: Reboot!
Step 8: Install Drivers
Now it's time to install hardware drivers. If you have a special driver for your motherboard's chipset, it should be the first one you install. As a rule, I like to install the video driver second, so I can see everything better for the rest of the installation. Windows will want to reboot after each driver installation. Deal with it.
Step 9: Install Your Anti-Virus
Step 10: Connect the Network
Step 11: Install Archiver(s)
Step 12: Update Your Files
Anyhow, for Windows, run Windows Update, and apply critical updates. Reboot. Repeat until there are no more critical updates. Incidentally, I would advise *against* using Windows Updates to update hardware drivers. I have had a few events that completely disabled a PC which I was able to trace back to a Windows Update hardware driver update.
For Linux (and OSX, really), use your distro's updater.
Step 13: Done!
If you installed Linux, you already have a fine repository of free software. Go ahead and install what you want!
If you installed OSX, most of what you want or need is probably already installed, but you're ready to add other things now.
If you installed Windows, the following links will take you to places to find free software for almost every need:
http://www.openoffice.org/ (also for OSX and Linux)