If you have personal gear such as a mask, fins, snorkel, and boots, bring them even if the dive shop provides rentals. Familiar gear always fits and performs better. One item I strongly recommend bringing is your own mask defog. Every diver has their preferred brand or formula, and I find the boat versions or communal bottles less effective. A foggy mask can ruin a dive faster than almost anything else.
Beyond gear, bring patience, curiosity, and time to listen during the briefing. The less you rush, the more enjoyable your first dives will be.