How do I disable monitor mirroring w/ LXDE Ubuntu 12.04

I'm trying to set up my two monitors so they're not mirroring each other.

I've tried going to

Preferences --> Monitor Settings

There, both of my moitors are detected, and I can Turn them on/off, change the resolution and refresh rate, but there is no option for enabling/disabling monitor mirroring.

Obviously, I'm missing something here.

I stumbled across the following commands:

xrandr --output DVI-1 --auto --right-of DVI-0

But I don't know what they do, and am afraid to try them because I won't know how to fix them if they mess things up.

Any help will be appreciated.

Thanks.

I'm running Lubuntu & here's some possibly pertinent information.
-Version-
Kernel : Linux 3.2.0-56-generic (i686)
Distribution : Ubuntu 12.04.3 LTS

-PCI Devices-
Host bridge : Intel Corporation Mobile 945GSE Express Memory Controller Hub (rev 03)
VGA compatible controller : Intel Corporation Mobile 945GSE Express Integrated Graphics Controller (rev 03) (prog-if 00 [VGA controller])
Display controller : Intel Corporation Mobile 945GM/GMS/GME, 943/940GML Express Integrated Graphics Controller (rev 03)
PCI bridge : Intel Corporation NM10/ICH7 Family PCI Express Port 1 (rev 02) (prog-if 00 [Normal decode])
PCI bridge : Intel Corporation NM10/ICH7 Family PCI Express Port 2 (rev 02) (prog-if 00 [Normal decode])
USB controller : Intel Corporation NM10/ICH7 Family USB2 EHCI Controller (rev 02) (prog-if 20 [EHCI])
PCI bridge : Intel Corporation 82801 Mobile PCI Bridge (rev e2) (prog-if 01 [Subtractive decode])
ISA bridge : Intel Corporation 82801GBM (ICH7-M) LPC Interface Bridge (rev 02)
SATA controller : Intel Corporation 82801GBM/GHM (ICH7-M Family) SATA Controller [AHCI mode] (rev 02) (prog-if 01 [AHCI 1.0])