What would you generally do in your first hunt seat equitation class?

[Tina]

On the flat. =)

...And if you could also add, what do you need to do to prepare?

[BeMe]

In Hunt Seat Eq flat classes you will normally be asked to walk,tort and canter in each direction and it is possible they will ask you to counter-canter, do a trot-halt transition or a canter-halt transition, sitting trot, hand gallop(which is basically and extended canter) etc,
I think that you shouldn't prepare for each individual show, just have good schooling and let shows be a test of how good you and your horse are. You should, however, be able to do sitting trot, counter-canter and several types of transitions. Good Luck!