Keith is correct: Scrivener (and the ability to export as a PDF) is all you need right now. A few things worth mentioning:
As you study screenwriting, you’re going to run into the page-count issue; in short, there’s a school of thought that says certain things in a screenplay should happen at a certain point (search for “Save The Cat” on this site and, again, lots of words on the topic.) Scrivener is not a perfect tool for knowing at a glance exactly where you are in a screenplay, so you’ll hear people in these forums saying that they do an occasional export to FD (or Montage, or whatever) to take a peek at page count. Easy, if you have Final Draft. Hardly worth the money, though, if that’s all you’re doing at this point.
There is an easy solution, however: Final Draft has a free demo that has no time limit. The demo is constrained in certain ways, but not when it comes to opening a document and checking page count. So I’d get that, as well.
As for Montage: It is indeed gaining ground on FD, and is indeed an excellent application. Wheere Montage suffers is that it lives in the gap between Scrivener and FD. It acts like Scrivener when it comes to organization, though it’s not as robust as Scrivener in that area (for my needs, anyway). And it’s not yet the production tool that FD is. I don’t agree that the Montage learning curve is any greater than Scrivener’s – I suspect what you may be dealing with is the unfamiliar script format. I will say, thought, that once you get through Scrivener’s learning curve, you’ll have the ability to execute any kind of project you’d like. At the end of the Montage learning curve, you’ll be able to write a screenplay, and that’s it.
So, why pay attention to Montage at all? Because Mariner is a very active developer. The leap from Montage 1.0 to 1.5 was enormous and right-thinking (and fairly quick). The Montage folks clearly get it, which says to me that they’re worth keeping an eye on.
I hope this isn’t daunting. It shouldn’t be: get Scrivener, download the FD demo for page count reasons, and you’ll be fine.
P.S. Also, the best way to truly get screenplay format is to read a lot of proper screenplays. That means screenplays in PDF format– not the html versions, and not the transcripts. Simplyscripts.com has a nice section here that lists “For Your Consideration” sites – actual studio sites that feature PDFs of scripts for Oscar consideration. Lot of good stuff there.