Becoming good at something takes time, persistence and a lot of hard work.  The ability to put together a story in screenplay form is no exception, of course there are those that pick up things more easily than others and are able to do this with no problem.   Those lucky, gifted few among us. 

Mostly everyone has to put in the time, effort and hard work to possibly see the fruits of their labor.  A little luck goes a long way too, the right place, the right time.

Finding time to do this especially if you’re holding down a full time job, family, bills to pay makes it that more of a challenge.  If you really want it, these factors and forces will push you to work that much harder.  If you don’t, you’ll give up the ghost and fall by the wayside.

Whether you set aside time to write once a week or every day, you must build a routine for yourself.  Perhaps you write one hour a week, perhaps it’s a couple of hours a day or even one scene a day.  Make sure that you write, and write consistently.

Make a schedule for yourself.

An hour or two before you go to work in the mornings.

Try and write before the kids come home.

Late night, early morning writing after the family has gone to bed.

Make a schedule and stick to it.  And you’ll see your writing take shape and you’ll have something to show for it.  That all important first draft.  Then crack on with the next one.

Keep writing.

N.B.  Make sure this is your time as well.  No distractions. 
And don't make up excuses as to why you should stop.  A lot of the time you'll try and walk away, don't, stay focused and write.