First, find wall, place head against wall, pull head away from wall, accelerate head to impact on wall sharply, repeat instructions...
Second (and less painfully), install a virtual machine manager such as VirtualBox on your system, then install Ubuntu Server 12.04 there and go back to "First" above until you get it working correctly. Once you have it running ok in a virtual machine, you will be ready to install it on your system.
FWIW, the only difference between a user workstation installation and a server installation are the tools and programs. You really do not need to install the server version itself unless you need some of the more advanced security features and such. In that case, DO NOT try to update/upgrade your system, but install the server software from scratch.
And in case you are wondering why I say what I do about this, consider that I am the principal performance engineer for a tier-one cell phone manufacturer and help manage performance and reliability for 4 world-wide data centers with a number of thousands of enterprise servers running in them. I run Red Hat Enterprise Linux (or clones) on all of those machines, in a VM on my company laptop, on my home workstation, and on my personal laptop. I used to run Ubuntu on my personal laptop, but got disgusted with it after 9.04 - that was, in my opinion, the best Ubuntu distribution ever, and it has (again, in my opinion) gone downhill ever since.
So, why do you need a server distribution? If you do, why not Red Hat Enterprise Linux, or a clone such as CentOS or Scientific Linux? Our company does support Ubuntu on engineering workstations, but not for server use. For that, we are strictly an RHEL or certified clone shop.