1. Most important thing is to write every day. Write every day. You have to write every single day, even if it's just for 15 minutes. Make time for it. You're only a writer if you write every day.
To phrase this another way: writing is a skill, like playing basketball or the guitar. If you want to get money in exchange for writing, and if you want millions of people to watch things that you've written, you need to be one of the thousand or so most skilled screenwriters in the world. This is attainable, but only if you are willing to invest a huge number of hours of practice. No-one, regardless of talent, is able to sustain a writing career without a ton of practice. The only way you'll succeed is if you practice as much as possible, as often as possible.
2. Watch this video:
https://vimeo.com/24715531 (original here)
Listen to what Ira Glass says: put yourself on a deadline, and do a large volume of work, in order to "close the gap."
Anecdotally, younger writers who produce three new original scripts each year seem to progress significantly faster than younger writers who think about writing a lot but only produce a script every couple of years or so. Don't take this idea too far - it's important to be thoughtful about your work, write things that are challenging to you and outside your comfort zone, and especially to practice the critical skill of revising your scripts and making them better over time. Still, if you think you might've been obsessing too long on one project, you're probably right. And if you're writing less than one script a year, you will probably be better served by producing more material rather than trying to make a signle script "perfect."
3. Read a lot of scripts. Have read every great movie, and every movie you admire. There are a ton of scripts available online. Try Lee Thompson's TV writing site, Simply Scripts and the IMSDB to get you started.
4. Read this: http://www.reddit.com/r/Screenwriting/comments/20pdf3/for_the_kids/
5. Read The Playwright's Guidebook by Stuart Spencer.
If you read only one book, this should be it. I think it will teach you more about writing drama than any screenwriting book - it's simple and straightforward.
6. It's not important to read more books, especially not how-to-write-a-screenplay type books. Many people I know, myself included, read too many how-to-write-a-screenplay type books early on. These books are useful to help you learn the structure of good storytelling; but they can also induce a sort of paralysis, where you begin to constantly second-guess your initial instincts when they don't immediately conform to traditional structure. This is to be avoided. However, since you're going to anyway, I will put book recommendations down below.
7. Listen to John August and Craig Mazen's podcast. Start with the free ones, but then pay for the old ones. It's incredibly, incredibly worth the $2.
If you want to write features, listen to a few of episodes of The Writer's Panel with Ben Blacker. If you want to write TV, listen to EVERY episode. Especially the early ones. And ESPECIALLY the ones with Vince Gilligan.
8. It's not critical that you get a film degree. It is critical that you learn, somewhere, how to understand movies and TV shows on a deep level - much deeper than, "that movie sucks." You should be able to articulate what you didn't like about it, why, what might have been done differently. Same with movies and TV shows you love. Everyone in Hollywood, or at least all writers and most agents and producers, can do this at an expert level. We all live in one town together and talk about it all day constantly and never get bored. Watch a lot of movies, think about them, and talk about them in a nuts-and-bolts way with your smartest friends.
9. Watch your three (ten, fifty) favorite movies or TV pilots/episodes with a pen and paper and a stopwatch. For each scene, write down what minute it starts and ends and what happens in broad strokes. (If you're doing this with TV shows, make special note of the act breaks (meaning when the commercials come on) - How many act breaks are there? How many minutes long is each act? Is one act shorter than the rest? Etc.)
Then, go through and highlight every moment where the main character's goal changes. You might notice in most great stories, this happens once about 1/4 of the way through the story, and again about 3/4 of the way through the story. (There might also be other big shifts at other points, and smaller shifts in many scenes. This is all great stuff to notice.) Eventually, teach yourself to recognize these turns as you watch any story.
This is the best method I know of for gaining a practical, applied understanding of story structure.
Free Online Resources:
First, check out Timeless Advice on Writing: The Collected Wisdom of Great Writers on brainpickings. This wonderful page, and the pieces it links to, are the sort of wisdom you can return to over and over for the rest of your life. Some of the advice may seem more esoteric, and less practical than books that say, "the end of act one should be on page 25," or whatever. And, trust me, I know all too well the seductive appeal of those sorts of rules and structures. The wisdom from the true geniuses is amorphous because they know how hard this is, and they don't want to bullshit you about it. Longer term, that kind of stuff is going to help you a lot more than the easy answers of the gurus.
Beyond that, the most practical nuts-and-bolts craft advice can be found in the following three places:
-- The Dan Harmon Story Structure Tutorials (especially numbers 101 through 105.)
It's free, takes 10 minutes to read, and is the best back-of-the-napkin guide to basic structure I know of.
These two articles by Randy Ingermanson:
-- The Snowflake Method For Designing A Novel
-- Writing The Perfect Scene
Randy has also expanded each of those two articles into short books, so check those out if you enjoy his advice.
And KM "Katie" Weiland's website, Helping Writers Become Authors.
-- Helping Writers Become Authors - Start Here
Her four best "articles," How to Outline Your Novel, The Secrets of Story Structure, How to Write Character Arcs, and How to Structure Your Story’s Scenes, are actually long multi-part series of articles. I think the content is really awesome and 100% applies to writing for the screen. (If you dig the articles, she's developed each of the first three into books and companion workbooks, and I think all of them are worthwhile.)
Again, don't read too many books. It's much more important to just write.
But if you're going to read books anyway, they might as well be good ones.
As mentioned above, read this first:
-- The Playwright's Guidebook by Stuart Spencer.
It's a great thing to read early on, because it gives a strong conceptual framework of dramatic writing, without bogging you down with too much structure.
Here are a few contemporary classics on the subject of writing that I especially enjoy:
-- On Writing: A Memoir Of The Craft by Stephen King
-- Writing Down the Bones by Natalie Goldberg
-- The War of Art by Steven Pressfield
-- Bird by Bird by Anne Lamott
-- From Where You Dream by Robert Olen Butler
(this one isn't really a 'contemporary classic', and it should be taken with a grain of salt; but it's also a kick in the ass and inspirational, so I put it on the list.)
Here are some screenwriting-specific books I recommend:
-- Making a Good Script Great by Linda Sieger. The best introduction to basic feature structure. Much more accessible and practical than "Story" or "Screenplay", the two so-called bibles of the industry (that are now seen as somewhat outdated).
-- Save The Cat! by Blake Snyder. Everyone in hollywood has read this by now, very good if you want to write the kind of movies Snyder did.
-- The Anatomy of Story by John Truby (especially the first 100 pages)
-- The Hero Succeeds by Kam Miller
Finally, here are links to Randy Ingermanson and KM Weiland's work, mentioned above:
-- Randy Ingermanson on Amazon
-- KM Weiland on Amazon
if you just moved to LA, or are contemplating a move to LA, and you want my opinion on internships/being an assistant/diversity programs/etc, check out this other thing I typed up. honestly, it's still a work-in-progress, but maybe you'll find it helpful anyway.