Most of these things come with the average distribution, so there shouldn't be anything you need to get. There's more, of course. Read the installation instructions carefully. There's no easy way to describe the installation process. Your distribution probably came with a copy, actually, so check before you run out and download it.
Type help ppp at the shell prompt.
I'm in no mood to read all of this. If everything worked properly, ifconfig will show your ppp0 interface as being up with your IP address listed. I warned you. I'm more patient. This will keep the riffraff out of your box.