As you've already been told more than once, you exit the server console by pressing Alt+F2 (or F3), and then log in with "root" as the username, and your admin password.
However, you really don't need to do that--you should be able to hook up your drives to the RAID card, assuming it's supported, boot from the CD, and install. That will reformat and repartition the drives for you.
Now, if you're really sure you want to do it manually, log in (as above) and do this:
# dd if=/dev/zero of=/dev/hdc bs=512 count=1
# dd if=/dev/zero of=/dev/hda bs=512 count=1
These commands assume that the drives are attached as primary master and secondary master, and will nuke the partition tables on the drives.