Well setting up a computer for remote access is a dangerous business because it opens a door that can be used by other people to do many things, including stealing information, proxy, botnets and so on...
First of all, the basics:
You must have a strong password and a good username. I recommend to create a separate account that you should use for remote access purposes with limited privileges (also known as 'limited user')
You must know your external IP. Keep in mind that if your are not paying your ISP for a Static IP, your external IP will change everytime you reboot your HSI/DSL modem. There is a way around this which uses a DDNS service.
You should have another computer to test the RDC connection. A laptop is preferred.
If you have a router, you must know:
Username and password for the configuration page.
How to forward ports
NOTE: In order to use RDC the target computer must be ON. I know this sounds silly but a computer that is powered on but 'sleeping' wont allow connections.
There are several tutorials on YT, I found this one to be interesting, it does not cover DDNS but it covers basic setup:
http://www.youtube.com/watch?v=z8BbrZqoXpY