How to remotely access lost mac?

I saw this article about an 18 year old who remotely accessed his stolen computer, downloaded a video, and posted it on youtube.(look up mark bao) Now while I am NOT intending on hacking anything, I want to know how to access your own computer (i have a mac and a windows, how to for both?). I am pretty sure it uses terminal on mac, though. My one problem is this- I only know my IP adress when connected to MY router. How can I access it, if it is perhaps connected somewhere else? To make a long story short, I basically want to be able to do at least the first part of what this epic kick-azz nerd did (I am a techie dude and I generally want to know a lot of stuff :):):):))