To set up an online shop or merchant app, you need to first establish a payment method and fill out payment details. This involves navigating to the payment settings, filling out general information like currency and timezone, selecting a default payment method, and adding branch details including payment info and notifications. You also need to set up shop settings such as delivery/pickup options, messages, and service hours before making your store live. Finally, you can add products and categories to your menu/catalog feature.