You need to know a bit about your hardware, specifically your graphics card and monitor.
For your monitor, find out its horizontal and vertical sync ranges, and the resolutions it supports.
All of this should be in your monitor's user guide or find it online for your specific monitor.
Then find out if your graphics card is supported by xorg, and which driver to use.
and/or search for xorg and your card.
Note that some ATI and nVidia cards have proprietary drivers available for Linux, if you want 3D acceleration for those cards you may wish to use those drivers. x.org has links to them, if you wish. The proprietary drivers may patch the kernel, review the requirements and installation instructions for more detail.
As a generic fallback, you can always use the VESA driver, it will work with virtually any card, though you may not get the best performance or resolutions that you will with other drivers.
Once all of that is done, type xorgconfig on the command line.
Follow all of the instructions in the menus that appear, entering the information as it appears.
When all of that is done, and you have told xorgconfig to write a new xorg.conf, then change directory to /etc/X11/xinit
Copy xinitrc.kde to xinitrc
(alternatively, you can also try xorgsetup, that will attempt to autodetect your hardware and set up an xorg.conf file for you)
Then type startx and you should be off and running.
If all of that is successful, you may wish to start X and some window manager when you start Slackware.
Do that by editing /etc/inittab and changing the default run level to from 3 to 4.
Then entry should look like this: id:4:initdefault:
Do that only after
you have X working well and you're happy with all of the X configuration settings.
Search this forum for more details about editing your xinitrc and xorg.conf files.