Hey good for you!!
There´s allways a first time for everything! so don´t panic!
The most important thing, is to really KNOW what the director wants. And in order to do that, you should ask him what he wants in each scene.
When does he want music?
How long should each music be?
Does he imagine any known music for that part? It could be a very good reference, not to copy, just to get the mood he wants.
Music should ACCOMPANNY the dramatic line, and so, you need to understand what´s happening.
And in the best case scenario, music should be a character by itself!
This can be achieved by having one or more recognizable THEMES and constructing each part of music derived from those themes.
You could make just a boring mood by placing some chords, but it´s better if your music has personality, and this goes BEYOND just having some cool chords.
Have a good theme, and USE the techinques of counterpoint, harmony, orchestration, etc... to your needs.
A good way to start, is to read a passage of the story and just start improvising.
It REALLY doesn´t matter if what come´s out at first isn´t good. EVERY idea is great for a start.
And lastly, dont MARRY any idea, let them evolve into whatever you are feeling they are evolving to. And if you have to sacrifice a theme ´cause it doesn´t please you, just do it, don´t be pressured by time. Remember what Napoleon said: "'Dress me slowly, I'm in a hurry"
Hope I was of some help.
Please let me know when you have something we can listen to!
Martin (argentina)