After installing Slack, you need to configure X logged in as root first as it doesn't do this during the initial install.
Configure X by following the steps. If all goes well and configures, at the prompt now setup a new user as you don't want to be logged in as root since its a security risk.
Add your new user following the steps it provides.
After creating your new user, logout root by typing exit and then login as your new user you created.
Now at the command type startx to start X, other wise known as the GUI. If you ever login and need to perform something as root, just type su - and it will prompt for root's password, then you can reconfigure whatever necessary.
Hope this helps, if you run into any problems, doesn't hurt to search this site for answers, can't find it, ask away.
Welcome ot LQ and Linux. And you picked the best distro to start with.