A friend of mine asked me for some advice on how to write a spec screenplay. Here's a few things I think every newbie writer should know.
1)Don't try to be too clever.
2) Write what you know. If you don't know anything, or only know very little about what you wanna write about, learn, read and research. Then write.
3) Outline EVERYTHING.
4) After you outline everything, throw it away (although go ahead and keep it in the back of your mind...it's a roadmap), and let your script breathe and go wherever it wants to go. So long as you know the story and characters, you'll be fine. Your characters (and the story sometimes) will probably surprise you.
5) Write everyday. Even if it sucks, and you can't think of anything to write other than Polly Wolly Weaseal a thousand times on that blank screen, write every single day. Set a specific time aside and know, no matter what, that from the hours of say, 3-4pm, you are sitting in front of your computer with Final Draft on and the little cursor blinking, with nothing else allowed to distract you.
6) Don't get too caught up in goals. If you wanna have twenty pages down by friday and you don't get it done in time, don't beat yourself up - just reset your goals, and make them more manageable. Smaller goals will make you feel more accomplished, and help give you the confidence to succeed in finishing, but the downside is, writing it, may take you a little while longer.
7) Once you're finished, you've only just begun. Rewrite your script. Rewrite it again. Show it to people. Then rewrite it again. A script is a living breathing world you're creating. If something doesn't make sense or slips through the cracks, someone smarter than you is bound to catch it.
8) Get a decent screenwriting program. Celtx is a free program that's similar to Final Draft which is the industry standard (www.celtx.com). (I would really recommend finding a copy of Final Draft tho...)
9) READ other people's screenplays, see what the format is like. Don't get too over involved in scene descriptions or people. Casting directors/directors like a good idea of what's going on in the script, but they don't need their hands held. White space in a script (meaning mostly dialogue due to formatting), is what studios like to see. There are plenty of great places on the internet where people post their scripts for review. Zoetrope.com is run by Francis Ford Coppola, and is pretty good for such things.
10) Have fun. It's not the end of the world if you find yourself halfway through the script and you hate the ending. A spec script is generally written for yourself, or whatever agent/studio you want to get their attention. It does happen that studios will buy specs, but generally only from established artists. What you're doing with your spec is creating a portfolio of sorts, so that when you do take your stuff to an agent, he can see that you know what you're doing. From there, s/he can either sell your spec if it's any good (and even then only a couple of those are actually made), or even better, get you jobs rewriting other people's stuff.