For partitions, it's very much up to you and your harddisk size. Usually you can create partitions with fdisk or even using linux's tool partitions during a linux installation. I believe that the most used partition program and simple to use is Partition Magic 8, which is NOT free. Take a look at:
For Linux partitions sizes, take a look at this tutorial.
They teach more or less how to divide your Linux partition. As for modems, I don't use a modem myself anymore. Make just sure that your modem is not a winmodem. (those crap plug and play that you cannot use/modify the jumpers). Ask at the store, they would help you with that. You could always check at your favorite's Linux distribution homepage if the modem you are thinking to buy is either supported or not. In fact, you should do it with all your hardware to avoid pos-problems.