About xrandr
From man xrandr:
Xrandr is used to set the size, orientation and/or reflection of the outputs for a screen. It can also set the screen size.
In my Debian GNU/Linux 7 I have found xrandr in the package named x11-xserver-utils.
Note xrandr is a tool to work with X server. It will be useless if you boot your OS with text console only and you want to keep it this way.
To install:
sudo apt-get update
sudo apt-get install x11-xserver-utils
How to know which package to install?
Check this question and its answers. In one of the answers I have created a basic guide to apt-file with xrandr as an example.
How to use xrandr to change resolution?
First simply invoke:
xrandr
You will probably see (among other things) some common video resolutions available to you, e.g.:
1360x768 59.8 60.0
1152x864 60.0
1024x768 60.0
You can switch to one of them like this:
xrandr -s 1024x768
Adding custom resolution is also possible. Read this.