Remote Desktop Connection to behind a router.

How do we remote (RDC) to a computer in a peer-to-peer network behind a router and a firewall?

I am aware there are quite a few brands of free VPN software for non-commercial use. Those for commercial use is quite expensive for subscription and probably not that “private”. I am wondering if we can use Windows Remote Desktop Connection or Remote Desktop Web Connection over the Internet in this scenario.

I suppose I need the WAN IP address. Then how do we get to a specific computer behind the router or even the firewall?