Hey GZ,
as mentioned, you will need to download TOR (the onion router), its free, but incredibly slow.
Once you have it, you have the problem of where to go? It's simply a search engine like google, just able to access 'onion' sites.
The best place to start is the hidden wikki. Paste this address into the TOR browser:
http://zqktlwi4fecvo6ri.onion/wiki/index.php/Main_Page
Its actually just a list of active sites. And to be honest, most don't work.
If you want to find a list of active onions sites, simply google (on the surface web) 'onion sites' and you will find a ton of them.
TOR itself, can access everything, not just onion sites. You could log into HC or FB through TOR. In fact, plenty of TOR users use it simply because they are paranoid about 'big brother'. Activity on TOR cannot be traced.
But really really slow.
