You can use the field Customer Journey Order Index (customer_journey_summary.customer_order_index) to figure whether an order was the customer's first order.

To filter the Shopify orders and only include first-time orders in your CSV/Excel export, simply add this filter under the Custom Filters section when creating/editing a Data Settings:

• Customer Journey Order Index is equal to 1

On the app's "Templates" page, there's also a template you can use called "First-Time Orders (Last 90 Days)" which already has this filter configured and can be customized further.