Without offending...you probably won't..NOBODY sells their first screenplay...Writing one is an art that not everyone can do[look in your local cinema for evidence..lol]
It is more than just having a good idea for the plot...it's about pacing it and making it readable to the Script Reader/Agent, and not cliche. having an idea for a scene or a character that inspires you to want to expand it into a script is the easy part..the hard part is finding the rest of the material to expand the initial concept into a full , working, pacy,DIFFERENT screenplay...
So keep it up and you will see that practice makes perfect!