Stay away from port 23. You and your friend will be hacked to high hell if you do. If your friend has Windows XP Professional, just connect using Remote Desktop Connection:
Start-Programs-Accessories-Remote Desktop Connection
He will have to enable remote desktop on his computer:
Right Click on My Computer - Properties - Remote Control - Enable this computer for remote connections and allow this computer to be remote controlled. <- Wording may be a bit off.
Other than that, you're done.
If he doesnt' have XP Pro, get a program called RealVNC
Furthermore, you're going to need to open up your ports to forward the ports on both of your computers. You sound young. I'm not talking down to you and I think it's awesome that you're getting into this, but fair warning: YOU ARE OPENING PANDORAS BOX. If you figure this out, you'll never be the same :)
Also, just know that if you do open these ports and don't have the actual computers setup with passwords (realvnc will need a password setup too), you WILL <-------- POSITIVELY get hacked.
I really think you should look at a more IT driven forum for such questions. This is not an amature question, and you'll want a pro to help you with:
Remote Desktop Configuration
RDP
Configure RealVNC
How to port forward
Google the above. Good Luck
10 Points, please.