My advice, having only ever written one screenplay, for a class, is to worry about format after you have written the thing. Just pound out the dialogue and whatever stage directions you want to include, divided into scenes. Then, after you have written the thing, then get a good guide to proper format and fit what you've written into that as scrupulously as you can.
That's how I did it. My screenplay wasn't long, about sixty or sixty-five pages. We didn't have to write a whole play or screenplay for the class, just a couple of scenes. But about five or six of us got into an informal competition to finish a full work after one guy in class, who was only eighteen or nineteen and a genius of a writer, wrote a cool screenplay for a horror film. I keep intending to get my screenplay out and round it out a bit and start submitting it, but every time I get close to sitting down to do it something else has to be done and it gets shoved to the bottom of the work pile again. Part of it has to do with the fact that I've lost the disc that it was on, so I'm going to have to completely rekeyboard it before I can start working on revising it, and I dread that.
But I'll do it one of these days. It's a good enough piece of writing that, with a bit of revision and a little fleshing out, I think it has a chance of finding a home somewhere. I doubt that it would fly as a theatrical feature, but I can see something like the Disney Channel or one of the other cable networks giving it a chance.