I definitely didn't do it alone.  I attended the New York Film Academy.  Having set deadlines and peers to critique my work were immensely helpful and motivated me to write and finish two screenplays, one TV spec script, and one revision of a screenplay.  Spend some time learning about screenplay format and structure before you jump in.  Find what writing methods work for you.  Everyone has different times of day when they're at their creative peak, different factors they prefer in their environment (silence, music, solitude, a public place), etc.  Figure out what yours are. If writing between 8pm and 2am works for you, then it would probably be harder for you to try to get up at 5am and force yourself to write. 

I studied screenwriting through the one-year
program at New York Film Academy. However, you don’t have to go to
school in order to write a screenplay. You DO need the following:
—A story idea that can be sustained for 90 minutes.
—A
basic understanding of how to format a screenplay. (You don’t need
Final Draft, but you do need to know where to set the tabs for Word for
character names, dialogue, action lines, etc.)
—A basic
understanding of plot and story structure. (For instance, by the end of
Act I, around page 20, we should know who the protagonist is and what
his goal is. Halfway through Act II, exactly in the middle of the
script, there should be an event that flips the script or changes
things dramatically. The end of Act II should propel us into Act III. Act III
is usually only about 15 pages or so. It’s the main showdown between
the protagonist and antagonist. After the climax, try to wrap things up
quickly by showing what has changed.)Study a few of your
favorite films. Analyze how they’re put together. Read as many
screenplays as you can get your hand on. (It’s always smart to read
whatever genre/format you’re writing in. Whether it’s poetry, short
stories, novels, sci-fi, fantasy, noir, etc.) There are lots of free
screenplay sites on the internet, so you have no excuse. Gaining
knowledge and exposure before you start will make the writing process
easier and the revision process as well.Remember to keep it
visual! Never reveal information through dialogue if you can do it with
action instead. Don’t have a character ask what time it is. Just cut to
the clock on the wall. We all love writing dialogue, but action and
story are just as important.Be persistent and don’t give up. In the end, persistence trumps talent.