The most important thing to build first in Google Analytics 4 isn't a report, but the framework for asking the right questions.
GA4 is fundamentally different from what came before it. As Brad Redding explained in a bonus episode of Honest Ecommerce, you have to start by asking, “What questions do I need GA to answer for us today, this week, this quarter?” Without that context, you’re just looking at data without purpose. The platform is more of a flexible, and initially empty, toolbox than a dashboard with ready-made answers. This initial setup is the most critical "build" you'll undertake because it determines whether any subsequent report is actually useful for your data and analytics in ecommerce.
Once your tracking is configured to answer your key questions, the landing page report is usually the most valuable one to focus on. Josh Piepmeier made this point on Ecommerce Coffee Break, calling it his go-to for understanding which pages are actually driving revenue. But the nuance with Google Analytics 4 is that you might have to build the metrics for that report yourself. Brad Redding also noted that even a basic metric like conversion rate needs to be created as a calculated metric. Building your first report is less about finding a hidden template and more about assembling the foundational pieces you need.