I want to write a screenplay. Where do I get started?

I'm currently reading Story by Robert McKee, The Filmmaker's Handbook, am obviously practicing actually writing, and am saving up for a good piece of software (recommendations?). What other advice do you guys have for me?

I've no formal education in filmmaking, and am mostly doing this for my own enjoyment (my prose sucks so I'm avoiding novel-writing). I'm under no delusion that it'll get picked up, though I'll at least try to do so. I've heard it's good practice to go through a screenplay while watching the movie, but I've no idea where to get a hold of those, or which movies would be the most beneficial.

Any other tips from those that have attended film school?

Apologies if this is the wrong sub to post this.