What desktop are you using? You will have to modify your /etc/rc.conf file:
That is a sample from my file - tailor your own file as you see fit.
# What display manager do you use ? [ xdm | gdm | kdm | entrance ]
Then if you issue the command (as root):
then reboot, KDM should come up as your graphical display manager.
# rc-update add xdm default
Of course this all depends on Slackware being similar to my system (which is not slack) - others can advise if the above method is not suitable for your distribution.
Regarding your resolution, you can either manually change it in your /etc/X11/xorg.conf file, or you can do it more or less automatically (how much automation depends on which method you choose) with a utility. Have a look at this to get some ideas.