GetPot is a project hosted at Sourceforge. There is a web page that provides download-able GetPot libraries for different programming languages:
To install GetPot, simply copy the file GetPot somewhere into the file system where your compiler can find it (/usr/include or so2). As a requirement you should have installed the STL-Library which usually comes with any C++ compiler distribution.
The easiest way to get started is to go through the example files and copy/paste the things you need into your own program. This way you learn GetPot in less than 10 minutes. The following text is mainly written, because every library should have a manual. It basically describes what you would understand anyway when you go through the examples.