An entry in fstab like the following should be sufficient:
/dev/sda /mnt ext3 defaults 1 2
1) It is unusual to mount /dev/sda as opposed to a partition of it (e.g. /dev/sda1, /dev/sda2). You might want to run "fdisk -l /dev/sda" to see what partitions are there.
2) /mnt is intended for temporary mounts (e.g. cdroms, floppies). You really ought to come up with a permanent name/directory if you intend to mount this automatically via fstab. You can just name something like /myraid by doing "mkdir /myraid" then substituting /myraid for /mnt on your command line and in your fstab.
You can verify the fstab entry without rebooting simply by typing "mount /dev/sda" and "umount /dev/sda" (or sda1 etc... see note about partitions above). If the fstab entry is correct it will get the rest of what it needs from there rather than requiring you to input details on the command line.