I've never used Slackware before, so I don't know exactly what you should do. Most automated Linux installers include partition\filesystem creation, but I don't know if Slackware has one.

For making Linux partitions in Linux though, general instructions are sort of like this:

# fdisk /dev/sda

(Change depending on which drive you want. 1st SCSI drive is /dev/sda, 2nd is /dev/sdb, etc. Same for IDE drives but they use h instead: /dev/hda, /dev/hdb, etc.)

In fdisk, make your partitions. You'll generally want a boot partition (100MB), a swap partition (1GB+), and a main partition (whatever space is left). Set the boot and main partitions to type 0x83 and the swap partition to 0x82. Make the boot partition bootable. After fdisk, restart the computer.

You'll need to create the filesystems after this, but the instructions for this should really be in the Slackware installation manual. Actually, I would read the manual and make partitions the way they say instead of listening to me.

If you've never used Linux before, Slackware may not be the best distribution for you (I recommend RedHat for beginners, though Gentoo is fairly good and I can help you with it since it's what I use). Captain Spam uses Slackware, so you could ask him if you have specific questions about it.