I think the slackware book
http://www.slackbook.org/html/book.html might also be a good place to look for basic disk partitioning etc. Slackware is a very good choice for a web server as it comes with everything you need pre-installed and ready to go. I generally use fdisk for partitioning. All you do during install is:
fdisk /dev/hda (assuming /dev/hda is the harddisk)
and then type p to get the current disk table printed on screen. Typing m will give you all of the options you need. Typing w will write any changes you've made and exit. Typing q will just quit without making any changes.
This is another tutorial which might help:
http://www.insyte.uni.cc/