Once your PC is assembled and turned on, look for the 'press F10 to enter setup' (or could be DEL or F2...etc...) message. Do this to enter BIOS (Basic Input Output System) settings. Try autodetecting the hard disks, this usually works fine. After this set it so that the CD-ROM is checked for a bootable disc before the Hard Disk is booted. This will allow you to boot the slackware CD. When you're in, run fdisk /dev/hdx (replacing xx with the partition with 'a' for first disk, 'b' for second...etc) and make a partition to install linux on. Quit fdisk (using the 'w' command) then run 'mke2fs -j /dev/hdxx' replacing xx with the disk number (e.g. a, b, c, d) followed by the partition number (e.g. 0,1,2...) so:
mke2fs -j /dev/hda1
will convert the first partition on the primary master disk to ext3.
Now run 'setup' to install slackware.