well just look what the current GM's do.

they organize events, host GM tourneys ingame and handle ingame related reports.

to become a GM you actually made the first "mistake" you dont ask how you become one ;)

regularly they make a thread in the news board where they hire new GM's, alot of people will apply then but thats the best way to become one.
sure you can always apply for it, but the chances are rather low that they accept you.