First, make sure you have the right kind of modem. Winmodems are lobotomized entities that require MS Windows to supply part of the processing that has been removed from the modem itself to make the modem cheap to manufacture. Winmodems do not work with Linux. Get the real deal. I prefer external modems myself, but you may prefer an internal.
External modems have the advantage that they can be moved easily from system to system.
PPP provides a temporary network connection to your ISP at low speed across the telephone line. During the time you are connected to your ISP, you are a node on their network and this is what gives you access to the Internet. Every node on the Internet has a unique IP number, called an IP address. Each IP number also has something called a subnet mask, which looks something like 255.255.255.0. One of the things your ISP's PPP connect provides you with is a valid, unique IP address.