If you want to GM your own game, write up and post the first game page explaining 1) the setting 2) introduction to the plot and 3) the rules. Then it is suggested (but not required) that you post it in the RP Kickstarter thread and call for players. Once you feel you have enough players and have perhaps gotten some feedback on how to better your game you may start it as a new thread in the main Role Play section. Your post will NOT be visible until approved by a GM. DO NOT make a Discussion thread until your game is approved. When approving games the RP Mods look for well developed concepts with clear instructions and rules. You will likely be contacted with some revisions and suggestions to be considered before your game is approved. It is also STRONGLY recommended that you participate in a game or two as a player before starting your own game. This helps you get the feel for how our community operates and allows you to build rapport with your potential players.
For more helpful tips and a lot of information on starting your own story check out Oasis Writer's excellent Roleplaying FAQ - this thread outlines RP terms and considerations you should make when forming your own game.