Welcome to Shopify and congratulate on your first sale. To get better on why your payment is pending, this writing: Shopify Payment Status Pending and everything you need to know will tell you why your payment is pending and what to do.
Hope it helps!
To piggyback off of what @Edward-Newgate mentioned, you can also view details of the order transaction within the timeline of that order. For example, if the merchant paid via PayPal, the pending status could be due to the fact that you haven't activated your account with PayPal or that the customer paid with an eCheque via PayPal, which can take a few business days to process.
Let me know if this helps!