First, find enough free space for the iso images.
Get Slack is the link. From Michigan you can use this
link.
You only need the first two CD iso images, unless you want
to run the KDE desktop environment. You will need the CD3
iso image for that. The 4 - 6 ones are source files, which
you don't need.
Or you can order a box set from
The Slackware Store, which also
includes
The Slack Book.
Then once you get the CD iso images, you will need to
burn
a CD to install with; or for alternatives, visit the
Installation FAQ.