1) Getting the OS
First go to http:/%2Fwww.ubuntu.com and get the latest version you're interesting in. They offer server version that they support for 5 years or so. I suggest one of those if you want to run a server. This is an ISO, in generate around 600Mb so be ready to wait a few hours... Even with a really fast connection, because the transfer will be limited by the mirror in most cases.
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From: http://linux.m2osw.com/install_ubuntu