If you need a GUI other than those which come work the system (twm, fvwm and cwm) then find the package, install it and read the accompanying pkg-README in
/usr/local/share/doc/"pkgreadmes"/[lumina,xfce4, kdeplasma,gnome,icewm,etc]
Follow the FAQ, man-pages, pkg-readmes, mailing lists and perhaps 3rd party guides for inspiration.