My advice: Write
No, seriously, you just have to force yourself to write as often as possible. If a good idea strikes you, write it in a notebook that you must keep close to you. Keep it by your bed and write down good dreams as well. Anything that sounds good, just write. I've used bits of dreams, small ideas, real situations and conversations, stuff I've read in the news, etc... all in my screenplays. If it fits, use it.
Formatting is the easy part now, since there are programs that do it for you. Start out with the free software Celtx at
www.celtx.com. It's similar to Final Draft, the industry standard. Trust me, it saves a lot of time when you're actually writing, and it's more reliable than Word templates.
Go here:
http://www.script-o-rama.com/To read screenplays for free to see how the pros write them.
Basically, you have your Scene Headings (Slugline), your action, your Character Heading, and your dialogue. Those are all the essentials you really need to make a good screenplay in the correct format. Once again, a program like Celtx can format those things automatically.
Scene Headings are where something takes place and will look like:
EXT. STREET - DAY
or
INT. BEDROOM - NIGHT
EXT. is exterior and INT. is interior, and it's all in caps.
Action is simply what happens. It is ALWAYS written in the present tense and active voice, like:
He runs to the door.
NOT
He ran to the door
OR
He is running to the door.
Keep the action simple and to the point. Never have more than 3 or 4 sentence per block of action. And don't be descriptive or poetic like you would want to be with a novel or short story; the best screenwriters hide their writing talent in favor of clear and consise action. The first time character's names are mentioned in action, they are in caps. An external sound that is not a person's voice is in caps (SIREN, WIND, HORN), although some writers don't do that, and it's okay.
Character Headings are simply the character's name who is speaking, and it's all in caps.
JESSICA
And dialogue goes beneath it. Again, screenwriting programs will capitalize it for you and align the spacing correctly for these things.
If you have someone's dialogue, then action, then the same person speaks, you write:
JESSICA (CONT'D)
Try to make dialogue sounds real. Don't write what you think sounds good, but actually speak it out loud and have other people read it out loud as well. Try to keep dialogue blocks short, as in, don't have dialogue run on for pages and pages without any action (unless you have talent with dialogue like Woody Allen or Kevin Smith, but even they break it with action).
They way you actually write is up to you, and different for each person. I usually start off by writing down a premise, a very general idea. Then I'll jot down more ideas as they come to me. I'll also flesh out my main characters - what are his or hers personality traits, what is their goal or desire in the screenplay, what is their life story, etc... Sometimes a detailed scene will come to me, and I'll write it out as a scene and tune it later. Eventually I'll try to write a brief treatment or outline. It usually has a clear beginning and for my screenplay, and a middle that is vague but gives a general impression for what I want to happen. And sometimes I just sit down and write in screenplay format, which can be creative and rewarding. If the mood catches me right, I can spit out 20 pages or even more a night. This is not common - you're usually lucky to get a couple. Sometimes, when I feel I have a firm grasp on the story and the plot and the images, I write a very detailed 5-10 page outline of every scene. Sometimes I don't. What I'm trying to say is, every writer has their own way, and for me, my own way changes all the time depending on what I want. You have to be flexible.
So yeah, you have to have good characters, and you have to have something interesting happen to them. There are other complicated things you could consider, like a graph of the plot. You need an inciting incident within the first 10 minutes, the thing that kicks off the story (think of the first zombie attack in Night of the Living Dead, or Luke seeing Leia's message in Star Wars, or Marion stealing the money in Psycho). You need a turning point 1/3 of the way through the story (Maximus is betrayed by the new Emporer in Gladiator, Luke joins Obi-Wan after his family is killed), which ends the first act. The second turning point kicks you into the last act (Lando betrays the group to the Empire in Empire Strikes Back). I know I'm using Star Wars a lot, but it has the classic structure!

Then of course there is the climax (Death Star!), and the resolution. If you're going for experimental, you can break all of these rules (although it's good to know what you're breaking beforehand). If you plan to sell to Hollywood, you need to follow the rules. So some good books to read are
Screenplay, by Syd Field
The Screenwriter's Bible, by David Trottier
Story: Substance, Structure, Style and The Principles of Screenwriting , by Robert Mckee
A lot of the shit they say is similar, but some is different, and they're all good to read. I'd also read Aristotle's
Poetics. It's old-school, but very good for the structure Hollywood still uses to this day.
You also have to think about your long-term goal. So if you don't plan on producing or directing these yourself, then you plan on selling them to others... which means you have to write to please others. Consider the business side - pick a type of movie you want to write and look up similar movies. Then see what companies made them, which will be your target prospective buyer. Don't make a gritty necro movie and try to sell it to Warner Brothers or Disney, and don't make a Fantasy comic movie and try to sell it to Troma. Know your speciality and follow it through. Know that your movie may very well never see the light of day, and accept that fact before you start writing. Thousands upon thousands of screenplays are written every year, and only a handful ever get produced... and only a handful of them ever get distributed. Your best bet is to write something in the no-budget category and try to "sell" it to a low-budget, no-name company who will actually make it. You might not get any money, but you'll get credit.
Remember to copyright your material through the U.S. Copyright Office, and you should register it with the WGA as well.
Also try for screenwriting competitions, which I am doing right now myself. Look for ones that not only have cash prizes, but also give coverage feedback to entrants, and also send the finalists work to agents and producers.
This is really just an introduction, an overview of steps you need to consider. Basically, you need to learn the format, learn the rules I outlined, and write, write, write... and then re-write, re-write, re-write. It's not easy learning to write in the screenplay style, and it takes practice. So if you're serious, it only takes your own motivation to do this. That's the beautiful thing, why I am focusing on screenwriting now more than directing. With directing and producing you need money, you need resources, and you need people. With screenwriting, all you need is a computer, motivation, ideas, and some know-how.