If you are going to get a dial-up service with an internet service provider(ISP), you will need a modem. There are 2 types of modems, internal and external. Internal modems are more popular and cheaper than external modems. Modems from NEWEGG.COM
Highspeed access requires a ethernet card, or sometimes a USB connection, for you to be able to connect to the ISP's provided external cable/DSL highspeed modem.
So if you are going with highspeed, you are set to go.
If your choice is dial-up, you will need to purchase and install a modem.