Where would be the best place to start with security?

I have (fairly) good knowledge of networking and how protocols work etc.
But what resources would you folks recommend to begin with, in order to get more out of AO and learn more about security, security issues etc.
What O/S would you recommend setting up on a home lab to play with.

I suppose what I am asking is where should I begin?



Thanks in advance

"variable_node"