My advice to not just #screenwriters but to #writers in general is this:

Yeah, listen to and read the advice of others more experienced than you, but write your stuff. Just get it out of your head and onto paper. Don't fuck about with form, i.e., margins and font and font size and word and page count. Get it all on paper. You'll have time to get it in the correct format and all *after* you've done all the other drafts and made it all work. (Trust me. You first draft will be shit. They are.)