Mike,
First, you use the newfs command to create a filesystem. For example:
newfs /dev/rz1c rz28
Second, do a file system check. For example:
fsck /dev/rrz1c
Next, the file system can be mounted. For example:
mount /dev/rz1c /u1
Lastly, you need to add a line to the /etc/fstab file so that the new partition will be mounted on the next boot. For example:
/dev/rz1c /u1 ufs rw 1 2
Hope this helps.