GMing is 90% preparation and 10% luck. Don't jump into starting it before you have your OOC set up, a few characters, and an idea of a story. The first few pages are what make or break the RP and you need to be able to build that world up within those posts. Granted that's where your players come in but you have to have a keen eye to say, "no, they don't know that information" or "yes, they can enter that area without setting off alarms."

The information above that everyone has posted is gonna be more than enough, more than what I can say, but you have to remain confident and expressive on what you would like to see. If you can show that you're determined, creative, and focused on what you want, then it will turn out as best as possible.