If you're able to, get your own pair of skates as soon as you can because they can take a while to break in, and the process can be painful, so you'll probably want that out of the way before you start lessons or join derby. If possible, go to an actual skate shop and get fitted...easier than having to guess and order online and possibly have to send your skates back.
In terms of what to work on, stance is a good place to start. Practice skating in a squat position, and make sure your chest is up and you aren't doubled over. Keep your feet about shoulder-length apart. You're going to spend a lot of time in that position when you start derby. :) It's going to suck a whole lot at first if you aren't used to it, but it does get easier. If it's really uncomfortable, you can also do things like wall sits at home to strengthen your quads. Work on finding your center of gravity and being really steady. Try skating on one foot if you feel comfortable. Really work on crossovers....although I'm sure you'll cover a lot of the technical stuff in lessons, as well.
Really, just being on your skates as much as possible is best...a lot of the skills just come naturally when you're comfortable on your skates. Get to as many open skate sessions as you can!
Hopefully that helps a little...best of luck! :)
"If your body is a temple, mine was a real fixer-upper." -- Chuck Palahniuk, "Survivor"