The most important thing you can do is memorize conjugation/declension rules. Over half of test questions are about that. Vocabulary is important too, but the list of words she expects you to know is pretty short and well defined, at least in Latin I/II and part of III.

Pay attention early on or it'll be difficult to catch up while everyone else is moving ahead of you. It's a class about understanding the Latin language and you would do well to not slack off at any point, but especially not early on when you're building a foundation.