• first of all: check if your kernel supports ext3!
  (most kernels since 2006 do that anyhow, so you should be fine) 
• optional step: touch /forcefsck
  and reboot
  (this will trigger a file system check, not such a bright idea on machines that are not KVM oder KVM over IP manageable, as something might come up during the fsck) 
• tune2fs -j /dev/sda1
  (this created the journal)
• edit /etc/fstab so that partition gets mounted as ext3
• reboot / remount