Edit - just saw the OP's second post, so my advice below is a little premature
Sounds like you just need to boot the computer up from the slackware CD or DVD and then once you arrive at the command prompt in the installer, you need to create some partitions - type
where /dev/sda is the disk you want to install slackware on - it may not be sda, so worth doubling checking if you're not sure before doing anything else. You'll need at least 2 partitions, one standard linux (type 83) partition and one swap partition (type 82). Once that's down you can go into setup, read the instructions as you go and, in most cases, accept the defaults.
It is a good idea to spend some time reading before you start - check out onebuck's sticky post in this forum, it's full of useful advice.