You should be able to do this quite simply from in the settings
This guide covers it quite well:
https://help.ubuntu.com/stable/ubuntu-help/display-dual-monitors.html (Latest Ubuntu Version)
As an overview you would go to System Settings, then Screen Display where you can turn the monitor on and off. Make sure to not have ticked "Mirror Display"
Hope that helps
Edit: For Ubuntu 16.04 - Copied from the docs (no longer available):
To set up an external monitor with your laptop, connect the monitor to your laptop. If your system doesn't recognise it immediately, or you would like to adjust the settings:
Click the icon at the very right of the menu bar and select System
Settings.
Open Screen Display.
Click on the image of the monitor you would like to activate or deactivate, then switch it ON/OFF.
By default, the Launcher only shows on the primary monitor. To change which monitor is "primary", change the monitor in the Launcher Placement drop-down box. You could also drag the Launcher in the preview to the monitor you want to set as the "primary" monitor.
If you want the Launcher to show on all monitors, change Launcher Placement to All Displays.
To change the "position" of a monitor, click on it and drag it to the desired position.
If you would like both monitors to display the same content, check the Mirror displays box.
When you are happy with your settings, click Apply and then click Keep This Configuration.
To close the Displays click on the x in the top corner.