Broadband solutions, like a dialup account, require a modem to facilitate the physical link, translating information from the computer into a signal suitable for the physical medium carrying your connection. However, unlike a traditional modem, cable or ADSL modems typically connect to an Ethernet port on the computer, which means that you must have a network card installed on your PC. Optus or Telstra should provide you with one.
When signing up for your connection, Telstra and Optus should give you a choice between a traditional PCI network card and a USB network adapter. Unless you're 100% confident that you can get the USB adapter working, I'd strongly recommend going for the PCI model. While I'm fairly sure the USB adapter is supported by Linux, it almost certainly won't be easy to get working, while the PCI card should be relatively easy.
A description of how to install and configure a network card is beyond the scope of this talk. Most distributions should automatically detect it, but if your distribution doesn't, or you have problems getting the card to work, there are a number of good documents out there to help you.