You will first need to enable quotas on the relevant filesystems (partitions).
Check the /etc/fstab file. Typically you would want to setup the /home or /var partitions with quotas. If you do not have separate /home or /var (or wherever you want to set up quotas) partitions, you can always enable it for the / filesystem.
The entry in /etc/fstab should look like one of these:
/dev/VolGroup00/LogVol00 /home ext3 defaults,grpquota,usrquota 1 1
LABEL=/var /var ext3 defaults,grpquota,usrquota 1 2
The first entry is for an LVM, while the second is for a regular partition.
Next step remount the filesystems with the changed options. Use umount & mount to reload the filesystem with the quota options. If you enable quotas on / or /var or /usr, you will need to reboot for the changes to take effect.
Use quotacheck to check & update usage / create the quota files (aquota.user & aquota.group).
Next use the setquota or edquota commands to enable quotas for the users / groups you wish to impose quotas on.
Use quotaon / quotaoff to enable / disable the quotas as needed. repquota to generate a report.