I've put links to the program and to various sources of documentation below. But you can learn by playing with the software, too. I've put some sample computations below.
The PARI/GP home page is the place to go for the latest versions, source, documentation, and so on. There is a self-installing distribution for Windows. For the Mac, see the information on the downloads page; you can either just build it yourself or get GP from the Fink distribution.
Documentation: