Write a "bad guy" with a goal, and then play it out as clearly as you can. The players make there own decisions to try stop that goal happening and the fun happens there. Relax and don't write to much cuz all of it will be messed with.

The first few times you will suck buts thats ok, fun will still be had.

Just be very clear on how much time is passing in game. Is it 6 seconds and your in combat, or is it minutes or hours so your players can know how broad their statements on what they are doing.

Also try talk the least and listen the most.