It's probably best to try one of the shorter scenarios for your first play - there's at least one with just three turns. But the critical thing is to start playing as well as reading the rule book. Indeed, best way to learn is to play. Beresford has posted some very helpful house rules (see the Files section here) with his own design notes here, which I recommend.
And on the subject of Vassal, I'm keen to start playing WBTS too, either by PBeM or live via Skype. My time zone: GMT + 1 (BST currently). I prefer live play but not always feasible. PBeM is slow but allows you time to play and read/learn the rules. Please PM me if interested. Let's get into this one!
I would try one of the later scenarios. The columns are smaller and there a lesser need to forage the map to death. You can concentrate more on the art of maneuvering (and coordinating) those columns.
I would also play a practice battle (set up your own). The mechanisms for playing out a battle are fairly straightforward and you shouldn't have any trouble with them. But what you need to understand is how absolutely devastating battles are to both armies. The Designers Notes make mention of this but seeing it for yourself will drive the point home more effectively. You don't fight unless you have to or you have a decided advantage (and you can keep your opponent from evading). There was a reason these guys didn't fight more than a battle or two every year. The design brings this out quite nicely.