Get Slackware. There are two methods, buy a copy or download your own.
Buying Slackware is a good option for a couple of reasons. The first reason is to support Slackware. The second reason is you may not have a speedy internet connection. If both of these seem like good things to you, you should probably go here and order a copy. A cheaper option would be to go here or a similar site and search for Slackware. They send you a copy for much cheaper, but your cutting the Slackware team out of some bucks. There are a lot of other sites which offer similar services, so search the web.
Download the Slackware ISO files. Personally, I like to use BitTorrent to get my ISO files. It's really fast. Anyways, if you want to use the BitTorrent Files, go here. If you want to download the usual way (it' going to take a lot longer!!!), go here and pick a server. As a note, there are two .iso files that you will need, the first two.
I used Windows ME to download mine. I used Nero to make the Cd's from the .iso files. If you don't have Nero, I'm sure you'll know what it is you do have to make Cd's, so just go with that.
After I've got my installation Cd's all made, it's time to reboot. While the computer is rebooting, I hit the delete key to enter the BIOS setup mode. You might have to press something else, I don't know. Just read the computer screen while it is booting up and it will usually give you a clue. I needed to change a few things in my BIOS to make things go smooth. First I reset the boot order. I like the order to go:
Next, I disable BIOS virus protection. I do this because I am going to be writing LILO to the MBR. If the BIOS virus protection is enabled (at least on my computer) you won't be allowed to write to the MBR. I also disable the plug&play OS option. Don't remember why I do that. Seem to remember reading it somewhere. Try it my way and see if it works for you. Save your changes, throw your installation CD into the drive, and reboot.