Slackware will offer to let you format the partition as you install it
You can choose a quick format, or a full format, or no format.
If it were me, and it has been a few times, I would do the full format. It isn't absolutely necessary, but I like to start fresh.
Congrats on your decision; I felt the exact same way when I loaded Ubuntu for the first time. Within 2 days, I had Slackware installed and never looked back. I love it.
Also, while you could use something like 'cfdisk /dev/hda1' or something similar, the easiest way to learn about commands like fdisk and cfdisk and parted (this is the command-line version of Gparted, as you would see in ubuntu) is to use the manual pages
In a console, typing 'man cfdisk' or 'cfdisk --help' will show you the help page for a given function.
So--- On with your Slackware install
and remember, it will offer to let you format the partition when you run the installer.
PS - I like Ext2 as a filesystem. You could use one of 2 other formats, but personally, I have had perfect success and ease of use with Ext2.