ubuntu servers - what do i need to do to keep it uptodate

Upgrading packages as you mentioned is the most important thing. A couple of tools that can make that easier:

• cron-apt, to notify you by email when there are upgrades to install; and
• unattended-upgrades, to automatically apply non-interactive upgrades

Ubuntu also has a security announcement list, but in my experience it is really just a duplicate of the package upgrades that cron-apt will notify you about. On the other hand it will provide a bit more information on what is being fixed (or what is vulnerable) in each security upgrade.

I installed munin on my servers so that I can monitor all sorts of things about them, including whether apt thinks there are packages to install.

If you want to just do the updates create a text file with:

apt-get -yq update
apt-get -yq upgrade

and add it to crontab for whatever timeline you want.