Firewall on ubuntu VPN ,lock it all up and tunnel in

I got my Ubuntu server running.
Need explanation how to secure the system.

First I like to know basic hierarchy how things get stacked up and
what software does what.

Firewall.
Need to understand how things work and what does what.

VPN
What is all the ipsec,....and other terms attached to this protocol.

Really just need someone to come down to my level of ignorance and
spoon me the basics as details I can research when i know what I'm
looking for.


u4davidatgmaildotcom