Read "man slackpkg" before using it.
This utility is intended mostly for official Slackware packages.
Slackware tools to manage packages : pkgtool, installpkg, removepkg, upgradepkg, makepkg, explodepkg.
Learn how to use it with "man <command_name>".
A slackware script is a script used to package a piece of software in such a way that it can then be managed with aforementioned commands.
To learn how-to use a slackbuild read the FAQ and the README @
http://slackbuilds.org.
Two rules of thumb:
1) Make a full install, so that you don't miss a dependency.
Remember that the slackbuilds provided @
http://slackbuilds.org always assume that you did a full install thus only packages not included in a full install will be listed as dependencies for every specific package.
2) Never compile and install manually a piece a software, to avoid messing up your system.
Find a Slackware package for it coming for a trusted source. You can safely install packages made by
Alien Bob and
Robby Workman, two of the main Slackware contributors.
If no package seems available, look for a slackbuild @ slackbuilds.org
Lats resort, ask here or learn how to make a package yourself.