There are a couple of things to note.
Firstly, you cannot go straight from 8.04 to 13.10. You can however jump releases if you are using LTS releases (8.04>10.04>12.04). This is going to take a bit of time, so make sure you're prepared for that. Also make sure you have backed up your important data in case anything goes wrong.
Secondly, you will need to modify your repositories (if you haven't already) to point to the old-releases. Edit
/etc/apt/sources.list and change every
://au.archive.ubuntu.com/ubuntu/dists/natty/main/source/Sources would be changed to
sudo apt-get update and make sure it works without errors. If it does, run
sudo apt-get upgrade and then
do-release-upgrade to upgrade to the next major version. You can then continue to use
do-release-upgrade until you get your system to the version you want.
update-manager-core to be installed, so you may need to install that before upgrading.
Once you have finished upgrading, you should change your repositories back to the current list if they haven't been automatically, or otherwise just remove the old-releases entries if they are still in there.
You can see https://help.ubuntu.com/community/EOLUpgrades for more information on upgrading releases that are past EOL (End Of Life).