a) you need to forward port 8000, the method for which depends on your OS and router. please google or otherwise search for how to do this, as i can't explain for every possibility.
b)
To become an Administrator, create a file named `config/usergroups.csv` containing
USER,~
Replace USER with the username that you would like to become an Administrator.
from the instructions
here which you should have read when getting the source code.
c) assuming you've forwarded port 8000, visit your server at <IP here>.psim.us. contact user: Zarel on #showdown on irc.synirc.net to get a custom address for your server.