Hi,
To just boot into DOS you can just format a floppy on the PC and transfer the system to it, this is done by typing "sys a:" (if a: is your diskdrive, ofcourse). This does however not work on anything newer than Windows 98 (well, perhaps also with Windows ME?).
If you want to make a partition on the Amiga for use with PC-Task/PCX you need fdisk to partition it to a format the PC can understand.