If it's your first time, gosh, keep it simple because most of it will be about the mechanics of running the story and the type of RPers you get to do it.
Ask yourself what the best RPs you have done were like, and how the person running it dealt with managing the players. That should help you decide how you would like to approach it.
How will you start the RP
How will NPCs be dealt with
How will things like order of actions occur
How will outcomes of actions be resolved
How will you move from one scene to another
What will you do when the players miss the plot and miss the clues
How will it end, will they know they won
Now, in doing that you can choose to be as regimented or not, from using some rolling dice system, or something like Fate-14, and even free-form self determination. The idea (well for me) is to have just enough so that the focus is RP and not mechanics.
The story itself I would suggest keeping it simple to start with, and leave convoluted things for another day when you have the mechanics down. So start with very simple, small scenarios.
Remember you are facilitating the RP of others, and you are supposed to loose while providing a challenge
What is the starting point that initiates the plot
Why would the characters care
Make sure there are ways to progress the story
Avoid single points of failure, because the players will mess up and that kills your plot dead
Do some work on fleshing out the main NPCs
Be prepared to wing it on general NPCs
Be open to players coming up with things you never thought of and accommodate them as best you can (in fact welcome and reward it)
I find it useful to write a separate bit on "what is really happening" rather than what the characters see. This lets your plot/NPCs respond to what is happening, rather than you responding to the characters.
The RP on adventures happens a lot in the still moments between the activity, let the players enjoy that before hurrying them along (there is no need to rush)
There is much much more but really, do something very small for a few friends and learn. That's the best way. And these can be as simple as the quests in the game:
Take item A to person in other place
Go and retrieve item A from the bad folks
Help for someone on the road
I have to go to the ball and I have nothing to wear.. help!
The Bishmark has run out of flower! - what will the characters do
Go and unload the ship and report back what they are carrying
etc..
Hope that helps.