Hi! Welcome to LQ
This might sound horrible, but first of all I think you should reinstall. Slackware 8 is very old. 10 is the newest version. You need to configure X with xf86config before starting it. You will need to know your hardware, if you give wrong values you might damage your monitor for example. X is configured with xorgconfig in slackware 10 (which you should be using...)
To select your window manager, make the file (as a user!): ~/.xinitrc and write this in it (this example starts KDE):
exec startkde
Now startx should start KDE normally, assuming you managed to configure X correctly.