Have fun, pace yourself (it's intense!) & as for 'form' - try to picture making smooth circles with your feet - pushing AND pulling up. And wear pants or capris - the shorts I normally wear aren't very short, but they still ride up & it's annoying.
Oopsie, almost forgot. Get to class a little early & have the instructor help you set up your bike, you need to be sure your seat & handle bars are at the proper height.