Like a good salesman or lawyer who understands why they're talking, you (the screenwriter) will find yourself having more fun with the words in your screenplay when you know why you're writing your story.
Therefore, knowing your real agenda (the real reason you're writing something) is key.
You say you don't have an agenda. Nonsense.
Every good writer has an agenda - what you hope a movie audience will take away from your film.
As a matter of fact, if you don't have an agenda your screenplay will lack substance.
There will be no voice to your script.
And you want to have a voice, so... how do you find it?
Is it all about practice? Practice does help, but there is something else you need to do.
How do I speak to people?
How does the tone of my voice sound when I talk?
Am I normally dark, moody, cynical, optimistic, ironic, dramatic or funny?
If you're uncertain how you sound, discuss a topic that you really care about with your friend or spouse and pay attention to you.
Did you hear your voice?
That's the trick.
Listen to yourself talk to others in real life (opposed to the made up characters in your head for the script you want to write).
Then, once you have a sense of who you are, all you need to do is understand why you're writing your movie script.
And when that happens, I promise you, you'll find the confidence to write your story the way you want to write it.
You will discover your original screenwriting voice and personal style.