From a beginner to a beginner: My advice, is atypical but it works for me. I started writing my feature film in April. I now have over 12,000 words, and a solid plot that I'm very satisfied with. All I need to do is finish up dialogue and a few small things and I'll be finished with the script.
First, screw all that technical crap. You can add or revise that later.
What is most important, is that you write as it comes to you. If you suddenly wake up at 2:00 am with a great idea, don't dismiss it and say "oh, I'll add that in tomorrow." GET ON YOUR COMPUTER THAT INSTANT AND WRITE IT DOWN.
Second, do your research. It's best to write about things you have interest in. If you don't already know a lot about something, then suck up as much information as you can. Google and Wikipedia are your friends. EVEN IF YOU DON'T PLAN ON STAYING 100% REALISTIC, FIGURE OUT WHAT IS REALISTIC FIRST, THEN WRITE FANTASY. You don't want to give a James Bond-type character a flying car trying to out-think creative bunches when they already exist in the real world.
On organization, well I started with a general idea idea for my story and then I filled it in with scenes as I went at random. Then I played "connect the plot". From there, I developed characters, backgrounds, organizations. You need to know everything about your film and the characters in it, even the things that don't make it to the script or screen. Your story may take place TODAY, but your character is going to be impacted by what happened in their life YESTERDAY. As the screenwriter, you have the sole responsibility of having that knowledge.
Finally, THE MOST IMPORTANT PART OF WRITING; PUT YOUR HEART IN IT. IF THIS IS JUST A SMALL HOBBY OR A DREAM THAT'S ALL IT WILL EVER BE. YOU HAVE TO GIVE IT YOUR ALL IF YOU WANT TO ACHIEVE ANYTHING.
In my personal experience, writing requires a LOT of sleepless nights. I figured out how dedicated I was to writing my feature when I woke up one day and my script had misspelled words everywhere. (I'm an extremely big language buff so this is rare.) I realized that it was a result of sleep deprivation in an extreme form.
Now I'm used to it. Sometimes I physically cannot sleep until I finish writing what's on my mind.
Hope that helps.