I think the biggest things you need to know on the horse are forward and relaxation. As an intro rider (I am an Intro rider on my IDA team, I took a 3 year break from riding and even though I was schooling first and second level tests at one point, I could definitely use the practice at a lower level again.. taking breaks is not good for your riding career!) you do not need to worry too much about collection. Judges will not expect it from you, and if you focus on it instead of what the judges are looking for, you could end up hurting yourself. Look for your coefficients for Intro Test B - Submission (acceptance of steady contact, attention, and confidence).. Basically relaxation, the horse should be relaxed, forward, alert while maintaining a steady contact.
Also your position is going to change, you are not as forward in your seat. You must straighten your back, hands above the withers, elbows bent and close to your side, leg long with heels under your hips, shoulders above your hips.. I had a hard time (after my break) sitting back, but you can feel an immediate change in the horse when you are leaning further back, it really helps them sit back on their haunches and raise their topline.