Probably ideal to setup a dynamic DNS service so you don't have to remember your
WAN IP address constantly.
I use
No-IP though I've also used
DynDNS both work the same, just different domain name choices.
On your router you should first give the computer that's running Remote Desktop a static IP/DHCP Reservation. Then forward the port accordingly to that IP.
Though in the end, I think you'll be disappointed with Remote Desktop. Not really made for use over the Internet. I'd suggest VNC instead (many various "distributions" out there). Or the easiest, use
LogMeIn