Linux Guide - Setting Up a Dialup PPP Connection
site: http://jgo.local.net/LinuxGuide/linux-ppp.html
Setting up an Internet connection can get tricky, even frustrating, especially if you're a new Linux user. If your ISP is like most others, it will use Point-to-Point Protocol. I will first describe setting up and using minicom and pppd separately, and then using a chatscript. One can connect to the Internet with either of these methods; if one doesn't work for you, try the other. If both work for you, then you can choose whichever one is the most convenient.

Red Hat Linux PPP setup HOWTO
site: http://www.linuxhelp.net/guides/ppp/
For this document to work on your system you need to have PPP support in your kernel and have your modem installed on a serial port with a symbolic link from the serial port to /dev/modem. If you are unsure of the above information, please refer to the Linux HOWTOs for detailed setup instructions.

The 5-minute Linux PPP HOWTO
site: http://www.knowplace.org/ppp.html
This is a step-by-step howto for getting pppd working with an isp using a dynamic ip address.