wessago

start by collecting hundreds of ideas in notebooks or cards. dialogue, scenes, characters. mix and match. see what makes you excited. stockpile ideas, don't commit to actually writing a script until it's burning a hole in you.
then write the first draft as quickly as you can. don't worry about quality. just get to 'the end'.
put the script in a drawer and forget about it for a few weeks. work on something else in the meantime.
once you return to the script, you'll be reading it fresh. like a stranger. you'll immediately know what works and what doesn't. write a second draft using this new information.
get some notes from friends. encourage them to be honest, to say when their attention flagged, if they found it hard to read etc. then do a third draft based on this new information.
by now, the idea as it stands is probably about as good as it's going to get. now send it to as many people as you can. producers, production companies. don't be paranoid about people stealing your ideas. ideas are ten a penny--it's writers they want, not ideas.