well first do you have free unpartitioned space on the drive ?
otherwise your going to need to get an app that will allow you to resize your present partitions, to make room ...
but to do this you will fire up :
(where X is replaced of course by you to the proper letter)
after you make the partition, write the table and then :
mkfs -t ext3 /dev/hdX
(again replace X, and replace ext3 if you use a different fs)
and then make changes to your fstab to reflect the changes ... it will look something like this ... (remember this is false info, and you'll have to replace the /dev/hdX and fs type if need be)
/dev/hda8 /tmp ext3 defaults 1 2