To use the fdisk command, you must specify the device you wish to partition. In Linux, IDE disk drives are designated as /dev/hda, /dev/hdb, /dev/hdc etc., where hda is the primary IDE device on the first IDE channel, hdb is the secondary IDE device on the first IDE channel, hdc is the primary IDE device on the second IDE channel and hdd is the secondary IDE device on the second IDE channel. If you have SCSI devices, the naming convention is slightly different. To partition the first IDE device, you would use fdisk /dev/hda. To partition the second IDE device, you would use fdisk /dev/hdb. The remaining devices follow this pattern. Once in fdisk, you will be presented with a menu (use m for help) that will allow you to create, re-size, delete and change partition types.
Having said that, I'd also suggest that you consider another Linux distribution (perhaps Mandrake or Red Hat). Gentoo is very low-level, requiring a good understanding of what is going on during the installation/build process. I suspect the Gentoo distribution is not intended for first-time Linux users. Just my $0.02, of course.
Hope this helps.