I was using awesome window manager and trying to connect an additional monitor to my computer with a vga port. A friend recommended xrandr. A quick look at the man page and you'll find:

xrandr -q

will list off all the connected display devices. I see four listed: LVDS1, HDMI1, VGA1, DP1. The two that were connected were LVDS1 and VGA1. So then after some playing with setting the resolution I found '--auto' so now a quick:

xrandr --output VGA1 --auto --right-of LVDS1 --auto

Works like a charm. If you prefer gui applications look into arandr.