LogMeIn, GoToMyPC, and similar technologies install something on your computer that communicates with a server provided by that company. It's probably the best way to go ... but since you asked about RDC ...
#1. You need to tell your DSL modem to forward the port to your router ... which you need to tell to forward the port to your computer. Port forwarding is absolutely what you need to look at.
#2. This would be the public IP address of your dSL modem, in this case. Unfortunately, that changes from time to time unless you have a static IP from your ISP. Check out the dynamic dns update utility at DynDns.org for a solution to that piece of the puzzle. Additionally ... you get a domain name tied to it that way ... which can be cool.